In the US, these vendors:
http://OWC.net is still the most-recommended supplier here, because they have good return policies and a lab full of Macs. They pride themselves on their service, so they do not sell stuff that is so "difficult" it needs a lot of hand-holding to get working.
OWC sells their own private-label SSD drives, which are pretty good and reasonably priced, and you know they will work in your Mac.
Look for the "My Upgrades" link on their main page, and it will lead you to parts that will work in your specific Mac.
http://DataMemorySystems.com is less often recommended, but very close in service and real-world testing, and often a little cheaper for comparable products with the same terms.
http://crucial.com is often recommended because they have a great track record of supplying stuff that works first time, every time. Personally, I am not as fond of them because they use their database of what sales "stick" rather than a lab full of Macs to obtain their results. Their recommendations tend to be more conservative than others, and they sometimes discontinue parts for older Macs before other sites do.
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All of these vendors have a lookup program where you enter the specs of your computer, and they recommend the parts that will work under these terms:
"It works in your Mac,
or your money Back." (it costs them money to get it wrong, so they make certain they get it right.)
Also, a Lifetime Warranty is standard, and their added vendor stickers on the modules are all you need for proof of purchase if you ever need a return.
