I was visiting Dollar Tree just an hour ago and saw plenty of dinosaurs that were the right scale for 28mm, selling for $1 each. There were Triceratops, something like a T. rex, stegosaurs, and generic sauropods. I already have a large collection of more expensive models so I didn't pick any up. One suggestion from me is to pick up a couple Safari feathered dinosaur toobs, which have a nice range of raptor-sized dinosaurs. Two toobs will run you about $16 in the US.