Have you been over to www.simpletractors.com ?
That is a Simplicity (and some others in the Simplicity family) website.
You may want to check that out if you don't get help here.

Have you been to one of the large Agriculture supply stores. like Ag Supply, or Tractor Supply between the two I have found just about any thing I needed. I know the Ag Suppy here in Garner has litterally thousands of belts. One wall 150' long

If you don't have access to that type of store, have you tried the tractor repair stores.

Based on the model #990399 that appears to be a Simplicity Landlord 2012.

Google "Simplicity 990399" and you'll get links to a complete parts list with part numbers.

Drive belt for 42" mower deck is Simplicity part # 2108099

Spindle drive belt for 42" mower deck is Simplicity part # 2108209

Best to order real Simplicity OEM belts rather than a generic replacement that may or may not work. After all, the original Simplicity belt has lasted how long?

This post was edited by justalurker on Wed, Jul 10, 13 at 15:20