The main thing you'll want to look for in a monitor for digital art is that it has an IPS panel, as opposed to a TN panel (or a VA panel, but those are much less common). IPS displays have much better color reproduction and viewing angles than the other types, though they tend to be a little more expensive.
I personally use this Acer monitor, 25" at 1440p, and it looks really nice aside from some backlight bleed that you'll find on a lot of IPS displays, especially cheaper ones like mine.
Both Amazon and Newegg have a lot of IPS options for around $250 or less that might suit your needs.