I order from McMaster almost every week. As far as Tri-Clover stuff they DO NOT have the best prices on that sort of thing nor do they have a variety of Tri-Clover stuff, they have the most basic things. But on the whole McMaster carries everything else in the world and 80% of it is better priced than most other stores. You'll find the occasional pump or motor that's way more expensive but mostly it's pretty good pricing. P.S. McMaster has, BY FAR, the best navigation and search engine for finding parts than any other website in the world. It freakin' rocks. Plus there shipping and fulfillment is very quick.
Internet wise the best place is St.Pats and their prices are decent.
Locally, and I'm sure they'll ship, the most reasonable prices around here, especailly Tri-Clover gaskets (Viton and Silicon) is Stainless Solutions. Look at the bottom for a list of companies I've made a list of that carry Tri-Clover. Ebay has some great deals but most of the time my local guy is cheaper.
http://www.stainlesssolutions.biz/ (Local to me and the one I use with the best prices.)
http://www.peiequipment.com/ (Also local and has alright prices, but has lots of other things, such as hoses and can crimp them on to fittings.)
http://www.csidesigns.com/index.php
http://www.jmesanitary.com/Results.aspx?manufacturerid=100 (Expensive and very hard to navigate.)
http://www.matrixps.com/
http://www.partsdeptonline.com/
http://www.johnernst.com/
http://www.stpats.com/index.htm
http://www.wcbflowproducts.com/
http://www.csidesigns.com/alfalaval/fittings.php?gclid=CJ2PwY-M5aYCFQYLbAodIEkP2A
http://stainlessaccessoriesplus.com/default.aspx
http://www.pbstbeer.com/