Thrift stores would be the first option. Then pair that with a good fabric shop, especially around Halloween, so you can get skull things. The fabric stores will also have leather and suede and fake leather products.
Get to know someone who is reasonably priced to do tailoring and custom sewing for you, if you don’t sew. The fabric stores can usually recommend someone. Then cruise the garage sales on the weekends and flea markets and pawn shops. These types of venues almost always have scraps of fabric and buttons. Other kinds of shops that specialize in hookahs and other smoking accessories, usually also have funky jewelry and skull stuff on hand.
Right now, because it’s October, go to all of the stores that carry Halloween stuff. You can find all sorts of things from Target, Walmart the 99 Cent store on up that will probably have something that you can use, for later.
You can also look into taking an art class, either at a school or even at one of the craft stores like Michaels. Especially if it’s jewelry making or ceramics (where you could learn to make your own buttons).
And don’t over look old lace which can be found at most thrift stores and garage sales and estate sales. Looks good on both men and women.