Why are the originals/equivalent so hard to find? This should not be complicated...
From reading posts on the Internet, it appears that the original AGS fuses were discontinued by the manufacturer (Bussman?) but Cessna never officially published their replacement/equivalence. So even the A&Ps can't be sure what other fuses they can use as replacement. Most played it safe and ordered from Cessna and paid the exorbitant prices. That's my understanding of the situation.