Take a look at Kraken and btc-e, generally btc-e has a lower market price than the other exchanges so you may be able to buy Bitcoins cheaper in there.
Thats because there is more risk to send money there and to get it back
. They are reliable, but more risky than most exchanges.
You can buy big amounts on every exchange out there, there is nothing that can hold you back other than your sanity
and self control.