Many of the small model catapults built by Team Tormentum have equivalents that can be purchased as kits from Ron Tom's family of websites. Alternately, you can buy plans for any of these machines.

Trebuchets

The Stirling Warwolf Trebuchet

F2K - a modern trebuchet

TK3 Model Trebuchet

Mini Desktop Trebuchet

Onagers

Desktop Onager

Mighty Roman ONAGER

Ballistas

Greek and Roman Ballista

Lithobolos Ballista

Other Catapults

Petraria Arcatinus

Scorpion II