For a good List of what is available go to Wikipedia! I have just put my Favorites there but since I don't like certain types of models I didn't list them here.
Free Paper Toys makes it easy to look for a specific model to make use in a unit. They also have paper dolls.

CANON backup pages sometimes have the toys they deleted!

Building a good education

These can be used as art for just about any topic of discussion. Read all the info before you click on Canon because you are going to get side tracked there!

My Favorite is Canon but be careful they delete stuff rather than get a bigger site. They sent me the link when I registered my first Canon Printer in 1997, thankfully I downloaded all the old stuff to cds before they started deleting. Any way they are the BEST site for educational downloads they have tractors, mice, historic buildings dinosaurs and animals of all sorts so go check them out. You will not be disappointed!

Epson Printers have some too

Hp has a collection of paper tools too

Yamaha has motor cycles and Endangered animals.Not for Littler Fingers

Howtoons are very interesting! Try some of these little projects.

Paper Toys has several Social studies Projects.

Paper Zoo is relatively easy to do with little fingers.

Racepaper has Racing Autos

NASA offers some Space Models

Complex Instructions for s Sea Floor Model

Build a Pharaoh has some good instructions.

Disney has some Fan Created Models.