In the USA, MR Members most often recommend Crucial (use the link above) OWC (www.macsales.com) and DMS (www.datamem.com) for reliable guaranteed compatible RAM. There are also any number of deals which change day to day from Frys, Newegg and other discounters. With discounter etailers and bigbox stores, ou are generally on your own to determine the correct specification and compatibility.
DealRAM.com lists memory prices, however a company being listed in DealRAM is not necessarily a guarantee of quality, compatibility, or reliability of the seller. Beware, some companies play pricing games to get on the top of the heap at DealRAM and then try to switch you to their "Premium" or "Hyper" RAM which is more expensive.