At work, I buy plastics (and hardware and pretty much any kind of raw material) from McMaster Carr.. Really easy to navigate and they have tons of stuff. Not always the cheapest, and it's hard to link to individual products, but if you go to McMaster.com
and search for part number 1227T129 you'll find 1.5" wide by 1/8" thick clear acrylic for $1.10 per foot..
If you want to search around for other acrylic, start here...
They have a lot of stuff though, don't get overwhelmed!
Also, as another bonus, they usually ship long skinny stuff in nice cardboard tubes that you could probably use in cosplay construction...
Visit my buddy's blog, GTDebris.com!
Cosplay, comics, Music, TV, movies, all sorts of fun stuff.