I live in the U.S., and my employer provides excellent health insurance for me (which follows me around the world), so I have never had a reason to buy traveler's health insurance in the U.S. or anywhere. So I cannot recommend any specific company or policy (and it would be off-topic to do so even if I could), but google found a few options for you:
HTH Travel Insurance has a few options.
International Medical Group
I'm sure Google can find many more options, if one of these doesn't suit your needs.
Whether you can buy these policies may also depend on where you live. And there may be additional options available to you locally through a travel agent or local insurance company. But since I don't know where you live, I can't really address that.