Really Long XLR

I’d use wall/floor mounted plugs and run Mogami or Belkin. This way you can keep the cabling 100% out of the room.

Otherwise, make your own with Mogami or Belkin, shielded is a must at these lengths, and Neutrik connectors.

Besides Parts Express, Parts Connexion (for connectors) you should see what Markertek has.  They often have good discounts on Mogami.

+1 to what Erik listed. Since you're willing to build your own, order some Belden cable from Raw Cable, and Neutrik connectors from Markertek.

I would make my own cables from 16 awg Duelund DCA wire.  The wire for a 40' set will cost you $400.  A good set of XLR connectors is $100.  Sheath material another $100.  If you don't have silver solder, another $15.

Jerry

Mogami W2534 Neglex Quad cable $1.19/ft. from Redco Audio.

Neutrik NC3FXX-B and NC3FXX-B typically less than $5.00 each. 

I believe Mogami is the preferred cable for professional recording studios.

Look into Audio Envy….I had 30FT RCA, they use OCC copper which is way better than most OFC cables out there.