I bought my sons Oset 36volt of Graham Jarvis. If you do a search on google for his contact details I am sure he will be able to help you.
Like the other post shows, they are not a cheap piece of kit but, They are worth every penny as they don't intimidate the rider. My son is 4 and he loves his.
My next door neighbours son is 9 and he can wheelie it a little and he has only rode it 3 or 4 times max.
The one concern I had about the Oset was that my son has riding experience as he had a LT50 quad and a PW50 bike and I wondered how he would react to the silent power delivery.... He managed no problem.
I bought an extra set of batteries as well but the life on the fitted batteries is very good as long as its not running at full throttle for too long, I have slowed it down a bit now to give him more balance and I can get 1 and a half hours out of a set of batteries no problem.