There is no way to know if any rental car companies have unlimited mileage on minivans
at every location (or only some locations), all the time, none of the time or only some of the time. I wish there was a simple answer!
I checked all the major car rental company websites... none gave any general information on mileage restrictions (for minivans or any other vehicle) country-wide.
I contacted reservation reps from 8 different car rental companies. None of the representatives knew if there were any locations within Canada that offered minivans with unlimited km at any given time... there's just simply no nationwide/company wide policy for this topic. Each said that you can only check for the company/location/dates that YOU WANT, to see if that option is available, and if not, what the price for the extra km would be.
There may be some companies in some cities that offer a minivan with unlimited km's, at certain times... but unless you're actually checking for that specific location, for that company, for that rental time period.. there's no way to know.
Unfortunately, the only question that can really be answered is, are there any Canadian car rental companies that have unlimited mileage on minivans ... in the city and location that I want to rent from, for the dates and times that I'm specifically looking for?
The same with the charge for extra km. It can vary from company to company, and depend on which location you wish to rent from --it's not something that is set for each company, country-wide.
You can really only search for availibility and pricing for the car rental companies that are in the city you wish to rent from... for the dates and times that you're looking for specifically. There's really no other way to find out in general of any specific Canadian car rental companies that have unlimited mileage on minivans.