Most dealers won't fit non OEM 'car' tires, as BRP seemingly prohibits it...
Over the years, more than once we've seen that if the name of a dealer who will/does do it for someone gets published/bandied about, it's not long before that dealer either loses their dealership or they quit dealing with BRP!!
So it really isn't a great idea to tell the world if you've found a dealer who will fit a real tire instead of the OEM rubbish - not if you want them to be able to fit one the next time you need a new tire, anyway!
Also, over the years we've seen hundreds if not thousands of Spyder Owners successfully running real car tires instead of the OEM stuff for what surely must amount collectively to MILLIONS of miles now, and amongst those, there's been people who've had accidents (some very serious) and insurance claims and those who've been booked/ticketed for other things
and over the years, there's been NOT ONE report of a liability issue or legal problem due to them running non OEM tires that are designed to go safely on the rims, so I suspect those 'Liability issues' your dealers are talking about aren't really a '
liability', but more likely they are aware of BRP's stance on this, and just don't want to risk it.
However, since the rims, the front rims especially, are truly just fairly standard CAR rims with 'J' type CAR tire beads (the early SMART Cars ran an identical rim, even down to the three studs; and some models even originally ran the same size tires too! Most just upsized the tire profile &/or width to run a more readily available tire size, tho, as do many Spyder Owners

) most Spyder Owners who want a proper tire instead of the OEM stuff find that any tire place can do tire strip & fits on them, especially the fronts, altho some of them can't readily spin balance them due to the rims having a smaller dia hole for the stub axle &/or their operators not being trained properly. Still, if you remove them and take the wheels in, most tire places can easily do the fronts, and the good places can do the rear too. The difficulty there is that you need to be able to get the wheels off yourself, or find someone who can do that for you, and the rear wheel isn't quite as straight forward to remove & refit as most car wheels! But it's not really all that hard, IF you can get down there, it just takes longer, and you need to pay attention to not messing up the fiddly bits like belt alignment & tension, the speed sensor and its spacers, etc. If you get a good quality tire that's been mounted
properly on the rear by a skilled/competent fitter, a tire that's already been checked and balanced and even marked so the tire fitter knows how to properly orient it on the rim to achieve the best balance/minimal run out etc, then due to the swing arm & thru axle support/design, there's not usually any major need for balancing, altho some owners still use balance beads or Centramatic balancers.
Some motorcycle places might be prepared to do the wheel remove/refit for you (at a cost, of course) but
since the Spyder's tires and rims ARE NOT motorcycle tires, few motorcycle places actually have the necessary gear to do a Spyder tire strip & fit. They
might be able to help with static balancing tho?!
So, I'd say
FORGET about finding a dealer to do it completely, and maybe do it yourself, or find some one/place,
NOT a dealer, who will.
Good Luck!