If you can't find a used one someplace like ebay then the only other source
for a factory service manual is the dealership parts department.

The best is your Toyota store, but that's also apt to be the most
expensive. Toyota's current price for the '98 Avalon at my dealer is
about US$120. The electrical manual is available separately.

If you're seeking manuals for an earlier model Avalon, don't give up
without enquiring of Walter Miller in Syracuse, NY
 http://www.autolit.com/ ,
and Bob Johnson in Framingham, MA
 http://www.autopaper.com/ .
I have had satisfactory dealings with both. Johnson concentrates on US
makers, but he also has some foreign marques.

For similar resources you might check the literature section in any
issue of Hemming's Motor News at major newsstands.
 http://www.hemmings.com

Brent
"Never lend your car to someone to whom you have given birth."
   -- Erma Bombeck

Your local Toyota dealer.