For foreigners who just arrived in Korea, the place to visit and purchase a T-Money card is any convenience store such as 7/11, CU, Mini Stop, or whatever. They are available in local airports.
As of writing, a T-Money card costs 4,000 Korean Won [around $3.00]. Card purchase is separate from top-up. There’s no minimum for the latter, however the limit [correct me if I’m wrong] is 500,000 Won [around 420 US Dollars].