Walking Eagle, I may have a solution for you. I don't know what your acoustic bass guitar is, or its size, but I have one of Glen Reid's lovely beastie's - its shape is like a giant A-style mandolin. On Glen's recommendation I got a gig bag that's designed for a 3/4 size 'cello. Excellent quality bag, well padded, fits real good and under $100 (at the time).

Source is:
George Heinl & Co. Limited
201 Church Street,
Toronto, Ontario, Canada   M5B 1Y7
Toll Free: 1 800-387-7858
Phone: 416-363-0093
Fax: 416-363-0053
http://www.geocities.com/geo_heinl/
e-mail: ghcl@idirect.com

I also found them pretty decent people to talk to and deal with.