Have mentioned on various occasions that art can be cheap on eBay if you avoid the big names and go for something from a lesser-known artist. You can get the 'look' without paying the price. These 1980s Lodewijk Lodewig signed and numbered op art prints are typical.
I'll be honest, I don't know much about the artist and the seller doesn't give a lot away. But the prints date from 1983 (although the 'look' is definitely one I would associate with the 1970s), each one signed and each one from a numbered series.
In a room inspired by the mid to late 20th century, they would work a treat. Even better, they are all at between £4.99 and £5.76 right now, so every chance of picking them all up for peanuts too.