Anybody know how to get the Ammo Stash card?

[𝕶𝖎𝖗𝖆𝖒𝖒𝖆𝖓]

Ammo Stash is the one that makes your secondary have infinite ammo, is it a supply line card?

[K~Mart_Security™]

Yup, just keep spending them supply points on the supply lines, and it will eventually unlock.
Your secondary will have unlimited ammo, but it takes a -20% reload speed debuff.