Suppose I want to build an AR380. Where could I get a barrel blank and who would be willing to ream it for me?
I'm planning to use a standard AR9 bolt and a custom adapter for Beretta 84 magazines.

You might try Green Mountain (gmriflebarrel.com) as they do 9mm barrel blanks. It might be worth trying ADCO (adcofirearms.com) since they do a lot of barrel work too.

I used to have an 84FS. It was a lot of fun to shoot and it's such a sleek and beautiful design.