When we lived in PA I belonged to a group on yahoo called free cycle.
Have you ever heard of it??
People post items they no longer need or want, if you see something you want you e-mail the other party back and make arrangements to pick it up (as long as it's not gone yet). It's pretty cool. The rule: MUST be FREE!!
Anyway a few months ago I got Austin a bike from free cycle, it was too big for him at the time (still kinda is) so I put it to the side. When I pulled it out over Christmas, he was able to ride it ok. When I picked it up the lady made it clear that it needed a new seat...Which it sure did, it had screws coming up and poking him in the butt!! I didn't realize this until after he was riding it. I thought it just needed padding, sorry little guy.
Now I will be honest to say that I only looked at 2 stores before I gave up and thought I would try to make one with supplies on hand. I used vinyl I purchased months ago and padding we that we bought the pack n play ( no longer in use). The padding was probably to thick but who doesn't like extra padding when riding a bike?