You can buy bitcoin from many of the Bitcoin Exchange services. Heres a few suggestions for each world region -
North America:- coinbase @ https://coinbase.com/
Europe:- bitstamp @ https://www.bitstamp.net/
United Kingdom:- coinfloor Ltd @ https://www.coinfloor.co.uk/
United Kingdom:- bittylicious @ https://bittylicious.com/
Australia/Asia:- coinjar @ https://www.coinjar.com/
Direct from Sellers - Global: LocalBitcoins @ https://localbitcoins.com/
Disclaimer: the listed companies here are only recommendations.
Magnr holds no responsibility or affiliation with making the above recommendations.
Please do your own research before making an informed decision to sign up with a service that best suits your needs.