A lot of confusion here. Spread betting is illegal in the US. OTC derivative products (i.e CFDs) can only be offered to accredited investors - basically anyone with net assets over $1 million and an annual income over $200k. FX is not so tightly regulated - pretty much anyone can open an account to trade that.
In terms of the most similar products you're allowed to trade, single stock futures are probably the nearest, although you won't find the depth of liquidity there. If you're after binaries look at the CME and Hedgestreet - both have listed binaries you can trade (as does the USFE, although the liquidity here is shocking). Other than that, traded options may give you the leverage you're after.
If any spread better or CFD company is willing to take a client from the US, frankly I'd steer clear of them. Breaking one set of laws may mean they're more fast and loose with others.