Just bought a used motorola i425 where can I

You cannot go to T-Mobile or AT&T for this SIM (Subscriber Identity Module) This particular phone is based on iDEN so you will need to go thru BOOST Mobile or NexTel to get a working SIM. I recommend BOOST mobile only because they have the $50 unlimited plan. You can also look at their coverage map to see if the area you will be in most has good coverage. Ciao, Luigi

Also, I do not endorse ANY cellular carrier. I only recommend BOOST due to the ongoing global economic crisis.

The SIM is based off of the network you are using, AT&T, T-Mobile, etc. You would purchase it from a store of theirs.