Your initial order comes with 28 gaffed cards so that PM can offer 'free flat pack shipping"...you'll need one card per performance.
Yes, refills are available.
No, it is not practical to make your own gaffs.
Rarely, can one make quality card gaffs.
Gimmicks, on the other hand, well, card gimmicks are made all the time.
Obviously, although the words are sometimes used interchangeably, gimmicks are not gaffs or vice versa.
However, let's not argue ;-)
Bottom line...it's a really fun effect.