Do I go for a domain if it's not available in .com or find one that works for both .us and .com?
Yes, I think if you're a business you should have one for the country your company is based out of, as well as the .COM version. It really depends on your needs.. We use .US, .COM and .NET.
And don't forget the .info and .biz extensions - they cost next to nothing, but it's nice to lock those up too when registering domains.