Where can I buy a "Boston shaker" in S.F. / any good barware supplier?
IKEA. Really. Glass and stainless with a rubber seal on the glass part.
They call it... Groggy, which is why I bought it in the first place. Great IKEA name.
It's a bit on the small side but does two glasses easily. I've been using mine for years.
And, no surprise here: Hella cheap.
4400 Shellmound St, Emeryville, CA
You might try Cask in the financial district. Their website lists a variety of boston shakers, including a simple mixing glass for $2 (http://www.caskstore.com/barware/15-oz-mixing-glass.html) which you'd combine with a shaker like this one for $7 (http://www.caskstore.com/barware/shak... ). You might want to call ahead to make sure they have those in the actual store -- so you don't get stuck buying their $99 (!) boston shaker set.
Some advice from someone who worked in bars.
Bars use a shaker set with glass and metal.
Reason why I wouldn't use the shaker set you're looking for is the smaller metal container which fits into the larger can get out of shape and you won't have a watertight seal. In a bar they get out of shape from being knocked to release, dropped or tossed around in the sink.