If you dont want to spend too much cash on jackets, check out some of the second hand clothing shops around chiangmai, I picked up a couple of good leather jackets, one made from Aussie lamb skin and the other real thick leather for 400 baht each, they are styled a bit like Indiana Jones's jacket but thier is enough skin on them to give you the protection you need, thier is also plenty of thin leather jackets from China but they wouldnt stand up to a slide down the road. You can also get a synthetic fabric jacket from some of the big bike shops up here with armour in them, one which impressed me had a Co2 canister and a toggle which attaches to your bike so if you have an off the jacket inflates like an air bag, but i dont know how long it would take to stop bouncing.
John