The Fairchild PowerTrench SSOT3 (super sot) is a small but powerful nfet (e.g. FDN335N and others). Digikey search will usually come up with a range of candidates once you home in on the specs and package.
EDIT: The highere power led's really like the constant current drivers. A fairly simple one is the ZXLD1350, which also has a dimmer input pin.
thanks very much, that does look like a nice device! In this case I want to use something that it pretty much a direct replacement for the p36's so no additional wiring will be needed. I will keep looking around