Most supermarkets have them these days, for about a pound. Also argos, which you can check local stock online.
All new sims come with pushouts so you get all three sizes in one.
If you joined in the last two years, its likely you have a 3 in 1 sim already, so try gently pushing at the centre -- it may save you a trip.
If it doesn't push out, then you need to use the sim swap procedure -- descirbed at https://community.giffgaff.com/t5/Using-giffgaff/What-is-SIM-Swap/ta-p/7320084 -- but all you really need to do is to log in to your old account before you activate the sim. Don't create a new account for it and if it asks you to buy a goodybag or credit, you're not logged in to the old account so stop and start again.