As to where to buy stones, there are a lot of places: Sharpening Supplies, Chefknivestogo, Hidatools, to name a few off the top of my head.
I agree with the suggestion to start out with a decent 1k/6k combo in starting out. I haven't tried the King 1k/6k, but I have tried a King 1.2k/8k with Ice Bear nagura and this worked well for me. One 1k/6k combo I do like is made by Suehiro. Hidatools also has an Imanishi 1k/6k which I'm sure would work well too. I prefer to hand-hold, so smaller stones are not a problem for me, with the smaller King being the most desirable for me here as to size.
In using a 1k/6k combo, it's best to try and get the best shaving edge you can off the 6k, then straight leather stropping. Once you've achieved this, then a Solingen dedicated linen strop with red tube paste on one side and white tube paste will really take things up a notch and can be used for touch-ups later, when the edge seems to be falling off.