Peter Mach is a great guy, close to Ottawa, and although he has not done any work on my instrument, I have heard great things about his work. His shoulder rests are fantastic, and I used one for a while in my dark days as a violinist.
Steve and Troy (and the rest of the team) at Heinl's are great and really know basses well. Steve makes great basses, Troy has made at least three and is a bassist himself, and they really know their stuff.
Mario Lamarre's work is amazing.
http://www.lamario.ca/index.php?lang=en His instruments have won gold medals, certificates of tone, ISB "convention favourite" etc. and apparently his extensions (integrated into one of his [DEL]pieces of art[/DEL] basses, or new additions to older instruments) are fantastic too.
There are some other good bass guys around, Ludi Pollack and Quentin Playfair in Toronto come to mind. I would do lutherie in Ontario/Quebec a disservice if I tried to name them all.
You should definitely seek out someone who specializes in or has a representation for doing good work on double basses. The above names are my short list from personal experience and the recommendations of players I trust with instruments in great condition.
As much as any well trained luthier should be willing and able, basses are often outside their comfort zone. Depending on who they have studied with and where, basses are often completely overlooked. The Violin Making School of America specifies seven violins and a cello, The New World School of Violin Making specify three violins, a viola, two instruments of choice, (bass not mentioned) and an exam instrument, the Chicago School of Violin Making specifies two violins, a viola, three instruments of choice with cello being optional, (bass not mentioned) and the North Bennet requires 5 violins, a viola, and a an instrument of your choice, with a bass currently featured on the website. Obviously a lot of makers don't go to "schools" but study privately with masters as apprentices, but
the idea that basses are optional is fairly common place.
I would venture that there is a reason why many of the well know bass makers (contemporary and pedigree)
focus solely on basses. Bow makers (especially German bow) occasionally fit this mold as well. From a repair point of view, they aren't a huge divergence from violin bows though. Depending on the style of your bass (flat backs come to mind, but there are other differences as well) it really isn't a "big" violin, but a combination of a big viol and violin, with a few oddities in the mix. It really is a different beast. A lot of the restorations I have seen are re-repairing things that were "fixed" ages ago, and I would not wish that fate on even the worst ply BSO.