But has no 10mm in stock..

So true; as before Underwood has absolutely no 10mm for sale on his website.  If you want Underwood 10mm, you'll have to buy it from Able's Ammo.

That stinks. They unfortunately don't have a whole lot either.

If you are a Facebook user, like their FB page; they will post what ammo they have in stock, when it comes to be in stock, most of the time.