If you decide not to make one, you could check Ebay, or find a WAHM site that might sell them a bit cheaper than Magic Cabin, or some of those other catalogs. I see some on Ebay quite regularly, and they do sell for a lot less.
I think its really much more special when you make it yourself, though. And don't stress if it doesn't look like the ones you see in the catalogs!!! Your son will LOVE it, and it will mean so much more because you made it!