I think a standard domestic cat PFD would fit a Chiuahua. I've seen them and PFDs for all size dogs at Petsmart in Arvada, right next to Office Despot just north of I-70 on Wadsworth.
Downriver Equip. may have them also.
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse