ebay has tons and craigslist may also. it is a normal rear axle for all wheel steer riding mowers.
most good tractors have AWS now but the ones to look for were the craftsman and like 4-5 off brand mowers. from the mid 90s to early 2000s
if you can find one it may be cheaper to get a full mower rather then the just the rear axle
also not to break rules or anything but theres a guy and a forum specifically about trucks like that. also using powerwheels and making mini go kart chassis for them.
http://www.mechanicalplanet.com/forum/forumdisplay.php?2-Main-Forum
and heres the guys youtube channel
http://www.youtube.com/user/chopchop426
may give you some good ideas and info. hes working on a video atm specifically about a DIY build
in a way its the budget version of the ones you looked at. and you can build them as you see fit. he takes a old power wheels that cost 300$ originally and spends like 1k in parts on them.
much better then a 2500-10k one that is 90% the same.
his red 4X4 is a 4wd quad chassis i think its 110cc or something one of those 500$ Chinese knockoffs