plander, DentonDoc is entirely correct.  Furthermore, there is a self-service setup at the Lac La Croix ranger station.  I made a trip thru there in mid-May a few years ago.  Just bring a form of payment (credit card number or Canadian currency) and a pen.

The reason for this setup is that no quotas are enforced until the day before the Ontario fishing opener.  

Technically, reservations are never required, but they are a darn good idea.  All you really need is an available permit if you go during the quota period from mid-May until late September.