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Thanks to the ease of creating webshops, many manufacturers will sell parts to you directly at low cost! This can be cheaper and easier than Digikey, or Mouser especially in larger quantities. I do it myself all the time
OnSemi "On-semi is the nuts-and-bolts part of the old Motorola. For $11 they will send you around a dozen different part numbers. For the lower cost parts you can get 25 pieces. As the price goes up the quantity reduces." - http://www.luciani.org
Maxim IC Maxim is great, and now you can even buy direct! Watch out for the lead times, and you may have to sign an export form.
Linear Technology Linear Technology has some of the best integrated switcher chips available (IMO :P) and now you can buy them direct. Login required, but its simple to get. They carry some parts that DigiKey does not, mostly the newer ones. Prices are good and they even do price breaks when/if you want to order quantities of 100 or so.
Microchip Direct Microchip sells directly and generally has better prices than their distributors. Single pieces often list for less than Mouser/Digikey/Newark charge for 25+. Shipping appears to be a flat $13 via FedEx. Out of stock products may have a lead time.
SupplyFrame Component Search Engine lets you find your part, it's datasheet and click over to the online stores for many component manufacturers including (SupplyFrame is free to use and there is no registration):
Pasternack Enterprises, Inc. Pasternack is one of those rare companies that sell only the highest quality specialty items with same day shipping. They sell special antenna, trimmer, and tool products - but where they shine is a one-stop source of patch/cable products: "any length, any termination, triple checked". This is the first place I look for odd and hard to find terminations in any configuration. They also have a wonderful connector identifier if your trying to match an odd connection and unsure what to buy.