Oh usually when I see 1700/2100, I think of Band 4 LTE used by AT&T, T-Mobile, Rogers, Bell but you're right, if it's 1700/2100 3G (AWS), it'd be T-Mobile in the US, and Wind Mobile and Mobilicity in Canada.
At any rate, there's not too many phones I know of that supports 1700/2100 3G since it's not that common outside of North America, and the models that are already offered by the carriers are variants that have been made specifically for those carriers, like these ones from Samsung from the past 3 years for example:
SGH-T889 Samsung Galaxy Note 2
SM-N900T Samsung Galaxy Note 3
SM-N910T Samsung Galaxy Note 4
SGH-T999 Samsung Galaxy S3
SGH-M919 Samsung Galaxy S4
SM-G900T Samsung Galaxy S5
There's a not-so-new list of AWS phones here (
http://en.wikipedia.org/wiki/List_of_AWS_devices) but I've found that only a handful of the larger manufacturers have models that support AWS. As for phones that run on that spectrum beyond what the carrier already has, the only recent one that comes to mind is the Huawei Ascend Mate 2. Of course, there's also all the Nexus phones.
The other reason I asked which carrier OP was thinking of using is due to the fact TMobile itself is refarming a lot of their urban areas to 1900, so 1700/2100 might be a non-issue there, and having 1900 on a phone is a lot more common than 1700 (for 3G).
Edit: haha, I see all these new comments only after I pressed Reply. Great suggestion there los33.