Only given options of EUR, CAN$, or AUS$. US$ was there before. Can someone explain?
I guess you are talking about the links you found on the right hand side on your wallet home page, right?
As others have already explained, blockchain.info itself is not an exchange.
Those sites you found (spendbitcoins.com.au, quadrigacx.com, cavirtex.com, kraken.com) are the partners of blockchain.info.
If you are buying bitcoin in US, you can try Coinbase, BitSimple, or localbitcoins.