As I understand it the figures come with it as a part of the educational play set area. If you wanted to purchase an additional set however I am sure that you could buy something supplemental. However given the size of the ride on I doubt you could fit a set of twenty in there unless they were very small, which would be a choking hazard for a child of the size the ride on is intended for.

Jam sandwiches in the video are the classic-my sister did that. So I can see the need to hide some things as replacements! I would just go and check out your nearest toy shop for some figures that you don't mind ending up in the bin and maybe keep the good figures that came with set till the child is a tiny bit older and less likely to flush them.

Keeping spares sounds like a good idea for both kids and husband's. I am glad that you found a system that works for you, but its a shame that so much goes missing isn't it?
 
I like the sets that come with this kind of thing, and normally it is possible to get spares through the manufacturer.