The Davis Mark III is the standard cheap sextant found on lifeboats etc. during the pre-GPS era. They're $40 new discounted from boating places ($50 retail) and you can get them in the $25 range on Ebay.

http://www.davisnet.com/marine/products/marine_product.asp?pnum=011

A replacement set of index shades is $6.00 direct from Davis (click the replacement parts link from above).

Edit: Looks like Celestaire has the Mark III for $34.00. Pretty good deal, I might have bought a new one from them instead of a used one for $25 if I'd known about it (but my used one came with a couple of navigation books, so I still did good). It's made of plastic and is not a fancy instrument, but it's ready made and should certainly be more durable than that $20 cardboard thing.