I've added 2 dealers to the price list who don't have web sites, but have ammo prices too good to not mention:

Kiser Munitions is a frequent vendor on the Equipment Exchange with positive member feedback in the EE feedback forum.

Collector Rifle is an established LE dealer in New York (often promoted here by member FAILSAFE) with a good stock of XM193 at a good price.  

I've posted their contact info in the "Recently on AR15.com" category in the Ammo Price List.

BLACK HILLS Blue Box 55gr FMJ - $240.00 delivered - Kiser Munitions - [email protected].
XM193 - $218/1000 + UPS shipping - Collector Rifle - 845-227-4100.