What kind or # of fuse do I buy for the Alde system?

They are two locations got Alde fuses/breakers...

There's the 'normal' breaker-box/fusebox (converter, under the bench seat) which has a breaker for the Alde 120v and a fuse for the Alde 12v.

There are also two fuses inside the Alde that protect the 12v negative and positive legs, separately.

You should be able to determine the size and type from this thread:
https://tab-rv.vanillacommunity.com/discussion/163/fuse-inventory/p1