Let Friends and Family Buy Your Materials
If your hobby produces things people enjoy having, let friends and family know that you are willing to "work" for materials. If your hobby is needlework, perhaps a friend who is expecting a grandchild would love for you to stitch a piece for the nursery, and would consider the cost of the materials to be a bargain. If gourmet cooking is your thing, give someone a shopping list, and prepare a meal at their house, or invite them to yours--a win-win.
Sell the Fruits of Your Labor
Many people with gardens end up with far more produce than they can use at some point, and many give it away. Ask the recipients of your largess to help pay for your expenses. If crafting is your thing, look into getting a booth at a craft fair periodically. The trick is to take on only as much as you can handle without it becoming work.
Is knitting your thing? Would your local craft shop pay you to teach once a week? Think about what you like to do and think about where you could give lessons. Perhaps your local community education program is looking for teachers, or the local store that sells supplies for your hobby. Can you trade golf lessons for green fees?
Write About It, or Make Youtube Videos
I know this is going to shock you, but most bloggers do not get rich from it. However, depending on your hobby, a blog could do a lot to support it. Book bloggers gain access to Advanced Reader Copies, which can be either physical books or digital. NetGalley and Edelweiss keep my Kindle full, and all I have to do is post reviews on my book blog, This That and the Other Thing. I get monthly mailings from Litfuse which offer me physical books. Catholic Word and Pauline Media send me pretty much any book they sell that I request. Most experienced book bloggers find themselves with daily offers of free books in return for a review. While Advanced Reader Copies are not supposed to be resold, and digital copies cannot be resold, it is easy for a book blogger to end up with piles of books which can be resold if desired. Check out It's Monday What Are You Reading, Mailbox Monday or The Sunday Post to become familiar with book blogs and the wide variety of review copies bloggers obtain. While other niches may not have the steady supply of review material that book blogs do, blogs that attract a readership can sell ad space, write sponsored posts, host giveaways and make money from AdSense or other advertising programs.
As far as blogging goes, my advice is different from what most people writing "how to blog" posts say. If you are blogging about your hobby (as opposed to blogging as a hobby) hoping to attract enough readership to get review copies and make a few dollars via affiliate links or Adsense, I suggest starting with a free Blogger blog. It will mark you as a not-professional blogger, but it won't cost you anything. WordPress may be more powerful (and it has tons more things you can spend money on) and your own domain name makes some people take you more seriously (or so I've heard) and is better for Search Engine Optimization, but self-hosting costs a few dollars every month and unless you attract a substantial following, you aren't going to earn that much--and you aren't going to get the following unless you really work at your blog. If your goal is to be a blogger, follow the blogger's advice on how to be a great one; if your goal is community and review copies, a free blog will work just fine.
If your hobby lends itself to demonstrations, demonstrate on camera and upload the video to YouTube. If you develop a following your videos can earn you money from the ads that play before the video, or you may be asked to use and advertise certain products.
I get about 4,000 pageviews a month on my book blog and about 2,000 pageviews a month here, and I earn about $4.00 per month from AdSense. I have more books than I can read, I secured a freelance writing customer and I've gotten some freebies like a nice set of mixing bowls, an Exergen thermometer, a dog toy and a grill brush. I'm a very small player in a very big world, but my blog supports my reading habit.
Find blogs about your hobby and chances are good that you will find giveaways offering supplies. You can even find giveaway link-ups. Many of them are run via Rafflecopter and can offer dozens of entries if you have the patience to look at everything they want to you examine but you can find smaller giveaways with better odds of winning, though the prizes usually have a lower value as well. I've won books, holiday decor and even an electronic prayer book.
Don't Forget about the Library
Yes, it is easy for a reader to obtain free (or shall we say paid for by taxes) books at the local library, but today's public library also offers videos, and access to a variety of databases. Instead of buying cross-stitch patterns, check books out from the library. If you want to build something, a book from the library may be able to tell you how. The library has cookbooks galore and magazines on many different topics. Instead of subscribing to Ancestry.com, go to the library and use their subscription.
Check Out Garage Sales and Secondhand Stores
Whatever you enjoy, chances are that other people do too, and that still others have tried it and found it wanting. As people de-clutter, all sorts of things make their way to the garage sale or to Goodwill. I know someone for whom the hunt for quilting fabrics is almost as much fun as quilting.
While most of our hobbies will never be truly free, a little creativity can make them much less expensive. What are ways you have found to save money (or maybe even make money) on your hobby?