The issue isn't the supplier. Play is very likely to be less than your local computer shop (who seem to generally be very expensive).
The issue is with the manufacturer of the RAM itself.
1) Make sure you get the correct stuff -- right speeds, etc.
2) Go for a brand like Crucial or Kingston.
Use kingston's website to scan your PC and tell you what memory you need. Then either buy direct from them or somewhere else, just stick to the same specifications for the RAM.
http://www.crucial.com/uk