Most bikes you can get there and then by just walking into the shop with the cash. and your residence certificate (from immigration). The only issue is price (as scuba says). You used to be able to get a bike at MSRP, or maybe slightly below, but now most add a farang tax of 8% or so. Asia Motors might get you a better price but you have to decide if it's worth the trouble. The only extras you will get is a flimsy jacket or cheap helmet (which you probably won't use). I asked for a lock instead. I have used the Mityon dealers on Thepprasit Road and opposite Big C Extra, both gave a good service (apart from price).