If I buy a non US droid could I use here in the us?
If it is a GSM device, that would put you on T-Mobile or AT&T depending on the frequencies it uses. You will want to look into which frequencies the phone operates on, as to which carrier it will work with.
If it is CDMA your options will be a little more limited. Sprint or Verizon I don't think you will be able to buy a device from anyone but the Carrier itself to get it to work on their networks. You may be able to get one flashed to Cricket or Metro if it is CDMA though.
There are no non-US Droids. Droids are a Verizon line of devices.
If it's non-US CDMA odds are it won't work in the US.
A GSM device would have to support the required bands and would need to be unlocked.