How am I supposed to set this as my signature if I can't even buy a signature?

Sucks for you.

Anyway, they're just not in stock. You have to wait until they get stocked.

Aren't store items randomly available? Wait until it's in stock.

I bought a bunch for -40 apiece.. would sell you one if I could

Sorries.

Or you can just "turn off the fun" and turn your minerals off, which will grant you your desired signature, as well as a few other neat account upgrades.

There are some on sale right now. Better buy it before its sold out again.