Free furniture - check out your local dump. You would not believe what people throw away. Pure profit-minus the products used!
I save a lot of my glass jars (from like salsa or peanut butter), take out containers and the cardboard parts of cereal boxes. I save my glaze / paint mixtures and watered down paint and use the cardboard boxes when I strip .. scrape them right off into that.
Miss-tints or "oops" paints are a godsend. Check Home Depot, Lowe's and Walmart. I just add my chalk paint additive and I'm good to go.
I re-use my Wooster Short Cut brushes when they aren't good enough for painting for my wax brushes instead of throwing them away.
Sealable sandwich bags are great to store brushes to save washing them out. I always have 3 that are rarely washed (white, cream and grey)
Vaseline for chipped, distressed look! Also saves you time!
Socks, old rags, chip brushes all get saved for poly or stain.
If you are an Amazon prime member, chip brushes and foam brushes are even cheaper.
I order cheap make-up brush sets for like $4 for 30 brushes from Wish. Great for detailing and have all sorts of textures.
I know this may be weird but I wash my food trays (meat trays) and use for paint trays. I also save every jar possible ( spaghetti, pickle, etc) to use for paint or water to stick my brushes in or mix paint. I recycle anything I can to use for my paint business.
Where I live we have a FREE site and I snag up a lot of paint that people get rid off. Not necessarily for furniture but for wooden crafts, stencils.
I put double stick carpet tape on a board to hold my cleaned hardware in place while I paint them. Keeps them right in place.
Dollar General (or other "dollar" stores) for kitchenware like rubber spatulas, tin foil pie pans/roasting pans, mixing bowls (with handles!), measuring cups (for mixing paint), etc.
I sign up for coupons at all of the stores I frequent. Harbor Freight does lots of free coupons with a purchase so I constantly get free tarps, microfiber cloths, and small tools. I have Hubby and both kids go through the line so we can walk out with 4 free items. My husband used to be in the Military so we ask every store that we go to for military discounts. I always submit for rebates. Hobby Lobby 40% off coupon and I have all of us go through the line. I will purchase damaged items for discounts. We did that with our butcher block counter tops and ended up spending only $350 for everything.
Ask for senior discounts - if you are old!
Use ketchup bottles from the dollar store to keep mixed paint fresh and easily use just how much you need. Dollar stores are the best for 90% of what I use.
Hit the thrift stores for kids t-shirts (usually 25 cents max) for lint free cloths. Usually nice and soft from many washes.
Have a craigslist alert set for 'table', 'dresser', etc in the free section. Saves me time picking and I actually get some amazing free finds. (You go to your Craigslist free section then do a search for 'table' or whatever and then save your search. That takes you to another screen where you tick a box to make it an alert. You probably need an account.)
Joanns takes Michelle and Hobby Lobby coupons. Not sure if it's vice versa.
Here are a couple of great DIY ideas from Dawn Jones: