If you want the +1 mag extension, you can get 'em from kel tec's web store, which is currently closed for another 4 or 5 days.
Or, if you're in a hurry I do believe Security and Police Equipment in Minnesota has a few in stock, at a reasonable price. Well, as close to a reasonable price as you tend to find on these little items, anyway. ;D
You may find it quickest to give 'em a call at 763-422-6741. They seem to have a good rep on the web and I've been positively impressed in dealing with them. I am not affiliated, just purchased a couple of mag extensions, and a spare mag, from them over the phone today.
If you're looking for the NON +1 GRIP extensions, there are a couple of kinds and a couple of sources.
Magazine Finger Extension Base Plates for the Kel-Tec P-3AT:
The Bersa (smooth) and Scherer (textured) style extensions need no modifications.
The NAA style extensions must be modified slightly.
Another option is the Kel-Tec 1-Round Magazine Extension which extends the magazine and also allows loading an extra round.
I've used both the Bersa grip extensions and the KT +1 mag extensions. Both help me with my grip. I much prefer the +1 extensions because they give me an extra round. They come with a new magazine spring from KT too, and are well worth the cost. Highly recommended, but make sure you read the directions carefully when installing.