You can get a T-Mobile SIM card from your local mobile stores, resellers or from the link below.
And, to check for T-Mobile network availability in your area, click the link below.
If you already have an existing account with a carrier like T-Mobile, then you can just get a SIM card from them and add it to your account. Just make sure that the SIM card is in standard size, it's unlocked and prepaid (pay-as-you-go) and has its own phone number.
NOTE: The PeTrack requires a standard size unlocked prepaid GSM SIM card.