T-Mobile has a promotional going on right now where you get a $25 card with any of their prepaid phones...for free!
If you're wanting to buy the Sidekick II or Motorola v330 there's a better option though. Amazon.com is selling the Sidekick for $300 out the door (free shipping, no tax) and with a $50 rebate from T-Mobile you end up paying $250 for it, $30 less than you'd pay for the device on TMo's website so you have $5 extra even if you turn around and buy a $25 card with the money you saved.
An even better deal is with the Motorola v330, which sells on there for $160. With a rebate it's $110, $40 less than the price on their site, $15 less even if you turn around and buy the $125 card.