I think you mean where and the answer is in a mini bike store.
One can buy a mini dirt bike from the local bike stores. Sometimes local community auctions also feature variety of mini dirt bikes for very reasonable price.
look on Craigslist Virginia
Often it is possible to buy a mini moto bike at a regular motorcycle salesman. You go to the salesman and ask him for a mini moto bike. Either he has one in his assortment or he asks his delivery man for a catalogue which he can show to you.
You can be any age to buy a mini dirt bike in the UK wether your 6 or 16 you can buy one but you are not allowed to ride one on the road at any age in the UK you would have to take the mini dirt bike to a dirt track and you can book weekly sessions with them so you can go like 3 times a week if you wanted to. So you are allowed to buy a mini dfirt bike at any age you just can't ride it on the road. i hope that's a help.
monsterscooter parts has them on the internet for $42
http://www.minipocketrockets.com/r32-4-stroke-super-pocket-bike/
best buy in Indian market one can go for is CBF STUNNER ...
you could buy a cheap second hand bike, or a Chinese mini dirt bike for no more than £600. but if you go for quality Japanese bikes you will pay a little over £1000. http://www.dirtbikefanatic.com/cheap-dirt-bikes.HTML
Buy a new one.
A mini exercise bike can be bought online at amazon and may cost around $150 for a new one. This however does not include the shipping price. Pictures and other information about the product is clearly labelled.
in wonder the same thing like could i buy a 125cc engine from a dirt bike and put it in a mini sprint