This print hanging in our den is by an artist called Metrofader. It's available online at Imagekind, a website where you can order prints in a variety of sizes and framing options (I ordered mine unframed and it cost a whopping $16.24. And it's even more gorgeous in person: the paper is thick, good quality stuff and the colors are rich with a velvety matte finish.) If you're an artist yourself, you can open a shop and sell your prints there, too. Some of my faves: Matasaburo Kazeno and Jason Brockert.
Thumbtack Press is another source of art prints. You don't get to choose the size print you'd like, but there are lots of interesting artists.
Left: Funny Houses by Sarajo Frieden. Right: Pistachio Ice Cream by Vanessa Tam.