As a long time Kingdom Hearts fan, this one really spoke to me. I've always loved the stained-glass art that appears in the game, and it translates really well to Undertale -- as you can see here with these prints by Marini4Art.
The prints are A3 glossy photo prints, and they're waterproof.
If you're not keen on this particular piece, but like Marini's art, they do offer commissions at various prices. So you can get your own completely unique piece of Undertale art at a pretty sweet price.