If you can verify the IAI Backup is the same as the AMT Backup ... which it apparently is ...
You can Google: Hoosier Gun Works On-Line Catalog:
They offer 4 models, AMT .380 Backup Magazines / 5 & 6 Shot capacity / Finger Rest - $25 -$30.
Isreal Arm's (IAI)
Modern Firearms, Quote:
"It must be noted that original AMT / IAI pistols were not noted for great reliability or durability, most models were "hit or miss" propositions."