I've bought these in Ikea and Woolworths, not sure which was the cheapest though and I don't ever remember seeing square ones. Perspex is quite easily cut though if your feeling like a bit of DIY!
I use a site that has what I think you are after - a website called www.imagey.co.uk - they have various perspex frames - one called the 'perspex float' looks to be what you are after I think. And it is available as a 20" square version from what I can see.