The exchanges get them from people who create accounts and deposit coins to later be traded or sold. Like all exchanges, they take a cut of all trades.
Think of hashes as encrypted stings that get "cracked/processed" to find bitcoins. A Pentium 7 processor should be capable of doing about 50 mega hashes a second. At the current difficulty, it would take it 46 years running 24/7 to find one bitcoin.
Thanks for your patience - last question - who hides the bitcoins in the strings so they can be mined?