I have '72 GM manuals I got for around $50. Try shopping "the Paddock". My GM manuals cover Chevelles, Camaros, Novas, Impalas, etc. and include the Corvette. Another source may be OPGI and Year One. Sometimes focusing on Corvette retailers for common items is not the best way to go. Not sure about 1978, though. Good luck.
I just looked-www.paddockparts.com. They only go to 1977 it seems, but offer a 3 book package set for $120! Go to Chevelle/Monte Carlo. Their Corvette link is only to 1972. Maybe try another area in there? Or, try calling their Customer Service-perhaps they can help. There's always google.com
My Corvette(s): 78 SA,78 Pace,black 79 and white 79, all L82s
EBAY! I got one in excellent condition for $8 plus $4 shipping. There's always a couple, both nice ones and kinda soiled ones.
Mike
Also, consider a 79 manual, just CORVETTE info. The 78 is ALL Chevrolet cars.
Did you try the NCRS at www.ncrs.org and click on the store logo. I don't think you can beat their prices and you can always call and talk to them. Great library.
Gary