Etsy and Storenvy are the biggest sites for people's custom shops as far as I know. Gumroad is also a great site to use, though it's known more for digital products (but I do offer physical stuff through there too!). Those three you'll find things that aren't produced through print on demand sites (Redbubble, Society 6, Zazzle, etc.), like enamel pins, buttons, charms, one of a kind/handmade items and so forth.
Some bigger artists host their own store site or have prints available through curated sites like InPrint.
Personally, I use Etsy, Storenvy, and on occasion people's personal sites, to do most of my non-mass-market shopping.