Having worked in auto before -- this is actually quite easy as it has more to do with the brand of the battery than the person who installs it... But only a little.
If you don't know how to install it yourself, go buy the battery from Sears or somewhere similar and ask a mechanic to install it for you.
Or, head on over to your favorite mechanic. Most will buy from NAPA or offer Diehard batteries. Those are fine. The last I read in Consumer Reports, NAPA brand batteries were fine and offered a good warranty.
If you live anywhere near Ballard/Fremont, I'd suggest visiting Superior Auto Service on Leary Way. They've been great to me and I'm confident they'd treat you well.