I don't know if this is still true, but last I heard lasers from 5mW to 2000mW (?) can be legally sold in the US if they have the necessary safety features (lock and removable key, 2-second delay on switch, indicator light) and if the manufacturer has a "variance" from the FDA. No penalty for the buyer except having it confiscated by customs. No penalty (yet) for owning one that's out of compliance.
I think Optotronics sells compliant lasers. After looking at their price tags, most people just pay their money somewhere else and take their chances.
Another way to get something high-powered is to order the parts separately and put them together. In that case (last I heard) there is no upper limit. However, the FDA has recently started cracking down on kits that contain all the parts.
Finally, there are people here at LPF who build lasers as a hobby, and put a lot of thought and care into them. Some of them are for sale.