Providing it's not a replacement for one which you've reported as lost of stolen (you'll have to sit tight for the one giffgaff post out to you), then your two quickest methods are;
Buy one from a local shop which sells SIM cards (most super markets, convenience stores, etc. sell giffgaff SIMs for around £1), or let us know your rough location (don't post your full address), as there could be a member local that can provide a spare one for you.
If it's a replacement for a SIM you've reported as lost or stolen, then you'll need to sit tight forbthe one which giffgaff will post out to you.
If not, then your options are;
Let us know your approximate location (but not your full address), as there may be a member local to you whom you can arrange a SIM with.
Or buy one from a local shop, practically most shops that sell SIM cards sell giffgaff SIMs for around £1.
These above are your quickest methods, though if a quick turnaround isn't important, order one from your orders page: https://giffgaff.com/orders/mgm and delivery should be within a few days.
Assuming that this is a replacement SIM, then you'll want to retain your existing number along with any credit or goodybags you have. To do this, you'll need to perform a SIM Swap, of which the following guide will walk you through it: