A lot of people use Best Buy or Wally World. Might want to call around let your fingers do the walking. I got mine though Customer Care so I didn't have to fight the crowds.
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The walmarts around here don't even carry the 8900.Best bet is T-mobile store.They got rid of the mail in rebates so you now get the discount instantly.
Definitely a corporate store because if something goes wrong you have all the records and if you buy insurance they always replace your phone if its been with an authorized retail dealer like T-mobile.
letstalk dot com has T-Mobile on sale (new plan - FREE, upgrade - $149). Bought my AT&T (FREE w/rebate) 2 weeks ago, got it in just a couple of days, free shipping, works great!