V.W.
A few things:
1- Check for recalls, if they are not recalled you have an option of any that aren't. The idea that you need the best is noble but not practical. I have used the cheaper kind and its always worked for me, and I know many others who do the same.
2- You can freecycle in your area and ask for one if you are very low on money, I know we are. You just need to be sure that its accident free and no defect.
3- In the same regards as 2, Craigslist, BayAreaKidsSwap which is a yahoo group.
4. Maybe sure that is it comfortable for you son, for your sake and his.
5. Know that of you are not satisfied if you purchase, within a small given time, of course and given that its in perfect condition you can take it back.
Hope this helps.
V.