If you want affordable art, try eBay. Ok, your average Sir Peter Blake print is rarely (if ever) going to sell for buttons, but if you want some interesting imagery that isn't going to cost a fortune, there's loads of it.
This striking Rafael Bogarin signed op art print, known as Jupiter 4, is a typical example. This hand-signed silkscreen print of a work by the Venezuelan artist was produced in 1981 in a limited edition of 295 and probably sold for a few quid in its day.
But right now, you can have this on our wall via eBay for just £6.50. An absolute steal. See the seller's other items too, plenty of similar works for similar prices.