Go ahead and buy it from somewhere like eBay.
Then you can buy the service from T-mo at the store
here it was about $50 with 300 minutes, but you can also buy the ones that come with a phone and stick the SIM in the Sidekick
I'd go for the $50 deal since you get minutes and aren't stuck with an extra phone.

I was thinking of getting the Sidekick 3 or the Sidekick SLIDE, I want it pay-as-you-go. Can someone tell me how I can do this?

I go look on Bestbuy.com and they only have the Sidekick I.D which i don't want. Then I looked on T-mobile's site and they dont have any side kicks in the prepaid section.

Thx.