No US exchange currently has Theta. Easiest way to get Theta is to buy another crypto like Litecoin or XRP and swap it for Theta at one of the trusted swap sites (I use Changeangel.io or Simpleswap.io).

Theta hit a high of approx. $15 in the spring however has consolidated in the $6-$8 range since then as the team continued working on various roadmap milestones, partnerships and it's NFT platform. All signs point to significant growth in 2022. Wouldn't surprise me if it is $100+ by this time next year.