I haven't found any really, they do make them for full sized drag bikes. But even if you bought one of those kits, I suspect that those kits are too long for a minibike so you would have to shorten them anyways.
In the past I have made wheelie bars for these out of mild steel tubing I bought at Lowe's, and rollerblade wheels. They work great and are fairly cheap and light. Current bars use drag bike wheels instead of rollerblade wheels, trying something different.