Netflix outside of the US, Hulu in the US or Amazon has them to buy for $3 each in HD, $2 in SD. I think Hulu has a free trial you can do.
[Or the myriad of "watch online" websites/piracy.](/spoiler)
Buy the series in the US right now on Amazon:
This is also how I get my Doctor Who. It's worth it and supports the people who make the show and I can re-watch whenever I want even if Netflix later takes it away.
I watched it on Amazon. Eight bucks for the season.
IMPORTANT:
You can buy the whole series on Amazon Prime Instant Video. Worth Every Penny!