A reprint of the factory service manual might still be available from Lloyd's Literature...They advertise alot in Hemmings...Their number(I think) is 1-800-BY-A-BOOK...I bought mine for my 85GN 5 yrs ago for about $120(I think it was)...It covers the whole 85 Buick model line...They can't separate, though...You have to buy the whole thing...Chilton's manual pretty much sucks(alot of times their photo illustrations are from a different make and model of car), along with Hayne's...Don't know much about Mitchell's, but know a lot of shop's use them...Personally, you can't go wrong with the factory manual...JMO...
Hope this helps...