Some restaurants make their own gyro meat from chicken, pork, beef maybe lamb. But they don't sell it retail/commercial as far as I know.
Phonecia Bakery will sell you sliced gyro meat from their spit for $7/lb.
Restaurant Depot carries many of the Kronos Gyrocones and precooked slices, pretty cheap, too. They also had a couple boxes of another brand (Windy City?) which I don't see listed in their database. These are their Kronos offerings (UPC, descrip, package price, case price).
07758917094 GYRO TITAN COOKED THICK SLICE 4/5# $18.04 $65.69
07758970045 KRONOS 1045 GYRO KONE BEEF/LAMB 2/20LB $83.46 $0.00
07758917071 KRONOS 1071 COOKED GYRO SLICES BF/LAMB 5# $18.60 $64.28
07758918111 KRONOS 1111 4/5LB CHICKEN GYRO SLICES $17.13 $63.82
07758966035 KRONOS 661035 HALAL GYROKONE 4/10LB $78.87 $0.00
07758966045 KRONOS 661045 GYROKONE HALAL 2/20LB $75.80 $0.00
07758917460 KRONOS GYROKONE 2/20LB BF/LAMB 1460 $77.76 $0.00
07758917450 KRONOS GYROKONE 4/10LB BF/LAMB 1450 $78.42 $0.00
I'm so glad I didn't buy the 40lb gyrocones for my own personal use. I settled for the 5lb box. Don't buy the "Titan" (their budget line).