I have seen them on peoples prams out here A and they usually have a 4-6yr old on them with bubs in the pram - seem to work quite well and the older kids seem happy than on the skateboard attachments - I saw one on a strider 4 and one on a bugaboo like style pram - they seem to sit slightly to the side so the child is not in the way of the person pushing etc. The ones I've seen have footrest but no pedals as such.

Can't tell you where they got them but given we are in the sticks I'm guessing a high end baby shop or online.