Micro Center, across from Oakland Mall, sells time cards for various games. You can give them a ring and see if the carry for STO. If they don't carry it already, ask them to order it politely. They are usually very accommodating.
32800 Concord Drive
Madison Heights, MI 48071
I just recently saw some cards at Ultimate Electronics. They're having a going out of business sale where I'm at, and I think they're doing the same all over the U.S. No idea if Detroit has one near you though.
Edit: Just looked at their website, doesn't look like they do, sorry.
Worst case scenario, you can always order them online. Amazon and Best Buy are usually good places to look. And don't trust the store availability on the Best Buy site. My local store carries them, but not according to the site. Hope that helps some.