You can buy steem and steem power directly on steemit. When you are logged in there are three bars on the top right of your screen, which are a button to a menu. One of the entries is "Buy Steem Power". When you click on it there is a nice menu that lets you buy steem and steem power directly.
I just bought 1btc worth this way.
I have now idea if that gets you a good price, but it sure is simple.
I just bought 1btc worth this way and the site tells me that my steem power up that i bought are worth less than 1/2 of one bitcoin. so be careful with this approach