Actually, T-Mobile roaming is available on more than just the other T-Mobile networks. T-Mobile is part of the Wireless Broadband Alliance that allows roaming through a number of major telecom provides, such as BT in the UK, Telstra in Australia, and NTT in Japan. I have used my T-Mobile account extensively in Australia for no additional cost. You can find a complete list of locations here

For i2Pass, as an individual you have to purchase through a reseller. i can recommend i2roam .