They typically are discovered and discussed on T-Mobile Reddit with quite a bit of discussion and information. It is a very popular forum and each carrier has a quite a following there.

Another point I'll bring up regarding the T-Mobile international data service is the use of IMS for dual sim phones. The current selection of dual sim phones is growing. Those who are considering or already own one can enable a feature that reliably gives free voice calling to and from US numbers to other US numbers anywhere (215+ Countries) TM has included service. This has been discussed at length on this and other forums, so do a search if this appeals to you for travel. Especially if you can snag one of the Free Lines I mentioned earlier or already have a spare line to use for the IMS function.