If you are looking for "Dispersed camping near Sisters OR", then the road west of town you mentioned, FS 1012 between 242 and 20, is a good choice.
As you head west of Sisters on 242, you'll find the turn about 5 miles out, just before the Cold Springs campground. About a mile down the road a left turn on FS 1018 will get you immediately into the best, most wide-open camping. Next best is on the right-hand side of FS 1012 just a couple hundred yards down.
If you inquire at the NFS ranger station in Sisters, youll find that they highly recommend this area for dispersed camping, as do I.
FWIW, Cold Springs CG is nice enough also, costs a small fee but doesn't really offer any advantage over the dispersed camping hearby.
Sisters City CG, as mentioned, is a nice little place about a block from what counts as downtown Sisters. It's very friendly, no longer cheap, but tough to go wrong there. Even if you don't camp there they offer free potable water to all campers, just pull up to the siding in fromt of the CG and you'll find a water faucet.
Sisters is overrun by a large flock of Urban Deer, so don't be surprised if they muscle you out of the way as they amble down the streets, going about their own business.
As for the Sunriver area, I know there is a nice Thousand Trails CG immediately sounth of Sunriver. They are not cheap unless you are a TT member, but they are conveniently located at the south end of South Century Drive, near the highway.
In the same area there are several Sno-Parks on South Century. If you can get the ODOT to give you a simple answer to a simple question (I couldn't), then maybe you could dispersed camp at one of those.
Good luck!