These guys will get you done with .22 (.224) in .308 sabots:
SABOTS for shooting (100 for $9.95 + $5.25 shipping = $15.20)
Gunbroker.com Sabots (100 for $7.50 + $3.00 shipping = $10.50)
JNS Supply - Sabot Accelerators (1000 for $40.00 delivered)
Sabots (200 for $27.95 + $12.00 shipping = $39.95; they also have the seating die for it)
As for a .243/.308 sabot, it doesn't appear that anyone makes one and if they do, I can't seem to find it.