Hi
some modellers, particularly the Eastern Europeans seem capable of building some truly outstanding models from paper. I tried it once an couldn't get along with the skills required at all- but some friends have tried them and loved the whole experience! The kits are quite inexpensive, but I'd recommend trying a free download from one of either steelnavy.com or papershipwright.co.uk. Both are representative of the modern generation of digitally designed kits and are probably a lot more buildable than some free downloads you come across!
Some superb free US civil war vessels can be downloaded too. Sadly the designer, who appears to have been determined to publish as many kits as possible without any charge recently passed away. The models appear to still be available to download. http://www.myspace.com/mmpapermodels
Marcle models' catalogue has a pretty good representation of what is currently available in this format. online here http://www.marcle.clara.net/
Finally, although they are European based, I foun moduni.com had a good selection of paper models in stock, and I have receive items from them very quickly overseas to the UK.
good luck. Hope you get along with them better than I did. I think I have just spent so long working with wood that I have got in to a lot of lazy habits that a card modeller must expunge immediately!!!
Will