Noting where you are in this wide, wide, world, would be appropriate when asking a question like yours.
Telling you to go down the street to "Jacks Computers, Orlando, FL" would be pretty stupid if you're in Australia. Eh?
Even getting something Mail Order from the US to UK or AU can get VERY pricey.
So first off, Where Are You? and what are you wanting to buy?
After years of getting burned by mail order companies, I've pretty well settled on Tiger Direct and "Computer Geeks".
They both have good products, with fast service and reasonable shipping costs. And both take "Pay Pal" for payment.
Tiger Direct also owns all the old "Comp USA" stores, so if there's one within driving distance of you, shopping in the store can be better than buying over the internet.
I have to place an order at least once a week, so I have quite a bit of experience with Mail Order.
For RAM memory, I deal ONLY with "Crucial.com".
Good Luck!