As I'm located in the U.S. I usually buy Bitcoin and Ether on Coinbase. I then transfer it to Poloniex to buy into altcoins or do some margin trading. Bitstamp is also one I've used which trades Bitcoin, Ripple, and Litecoin.

ADVICE : I wouldn't buy at this moment. We're seeing all time highs at least with Bitcoin. If you're looking to invest here are the prices I would monitor for these coins. In the end, long term growth is imminent so anytime you buy will be a good time. These prices will just give you optimal investments.

Bitcoin: $2400-$2500
Ripple: I think Ripple will still consolidate. Look to get in under $0.15
Litecoin: $38