Golok,
Typical stores that might have paracord are military surplus stores. FWIW, 550 paracord should typically cost you about $5-$8 USD (or less) per 100 ft bundle plus shipping. It is much cheaper in 1,000 or 1,200 foot rolls, which is the way I recommend buying it to get a much better per-unit price. It is typically $35-$50 USD per 1,000 ft roll ($3.50-$5.00 per 100 ft) + shipping. If you can't find it in your area and don't need 1,000 feet of it, you could buy a 1,000 foot roll of it, take what you need, and sell off the excess at the $5-$8 typical price to help pay for the roll.
Beware of the vendors charging more than the prices noted above. I've seen up to $10-$15 per 100 ft.
If you see "paracord" in colors other than black, white, red, or Olive Drab (OD) it is probably not seven-strand 550 cord. Realize that the imitation non-550 cord also comes in black, white, red, and OD. So check the cord out by peeling back the outer sheath a bit and counting the inner cords to insure that there are seven of them. If you are buying online, verify with the vendor that they are selling actual seven-strand 550 cord, not something else, before buying cord from them.
There are usually a bunch of auctions for paracord on eBay. WARNING: Be extremely careful with these auctions until you establish who is actually selling 550 paracord, not just "military style" cord or "parachute cord" which is not actually 550 paracord. The shorter lengths of paracord sold on eBay (20-40 ft lengths) are likely used shrouds that have been cut off of parachutes, so the condition of the cord is unknown. IMHO it is worth it to buy new 550 cord because the price isn't all that much different from these chopped off shroud lines.
MaineMilitary.com advertises their 1,000-foot rolls of paracord on eBay as the real 550 cord. (disclaimer: I haven't done business with them personally) They have a roll of OD 550 paracord on eBay now with no bidders at an opening bid of $25 USD + shipping. That equates to $2.50 per 100 ft, which is a very good deal!
Amherst Drop Zone carries 550 cord in black, white, OD, and red in 1200 ft rolls for $50 USD + shipping. (disclaimer: I haven't done business with these folks either, so can't vouch for their product or service)
http://www.drop-zone.com/catalog/show_products.cfm?headingid=97&catid=56&depth=1
Brigade Quartermaster has 550 cord:
http://www.actiongear.com/
US Cavalry also carries 550 paracord:
http://www.uscav.com/search.aspx?Search=550+cord&tabID=
If you don't need real 550 cord and want the lighter cord in bright easily-seen colors,
http://www.ubraidit.com/ has it in a bunch of colors.
