UK based there’s Alpkit, Apidura and Wildcat. US based there’s quite a lot, check out http://bikepackersmagazine.com/ – they have quite a few comparisons in their review section.
For my main bags (Handlebar, frame & seat) I use Wildcat kit, for the smaller stuff: fuel pods; stem cells, I use Alpkit. The Wildcat stuff is rock solid, but read the instructions carefully – there’s a bit of threading back and forth to do which is a fiddle the first time but once you’ve done it once or twice it’s quite simple.
If you don’t want to splash out loads of cash and are only doing one or two overnighters then you can strap the Alpkit Tapered Airlok directly to the seat/seatpost.
Remember to budget for helicopter tape – the straps will rub the paint on your frame otherwise.