You can purchase most off the shelf prepaid or Billed SIM cards and use them to operate a tracker.
However you would need to change the tracker settings as per tracker instructions so it can recognize the new SIM card. This involves changing the APN to that of the new SIM card. Shelf SIM cards are becoming less expensive and if you are using the tracker for a short term application they will work just fine. However if you need a tracker to operate over several years it will become very expensive as it would cost in the region of £10 per month which quickly adds up.
When purchasing a tracker for a long term application ensure it has a data SIM installed. It shouldn’t cost anymore than £3 per month if a data SIM is used which significantly reduces your outgoings. A data SIM in a tracker uses in the region of 10mB per month.