I am trying to make a push toy for a friend's 1-year old. I need to put some wheels on the bottom.

I found some casters that I can put on the bottom but I was thinking it would be better to have little tires so that it would push straight.

I can't seem to find any kind of tires that would be suitable. i have seen some but they are $8 each and I was trying to keep it down to $8 for all 4 tires. Any ideas on where to look or what kind of tire to get?

Thanks,
Greg