Here's a list of Ontario sled rentals that the Intrepid one made some time ago. There may be changes by now. I thought I had read somewhere that Horwood Lodge has rentals but couldn't find anything on their site :
Operators Who Offer Ontario Snowmobile Rentals
Baysville (Muskoka): Baysville Marina
Cochrane (Northeastern Ontario): Cochrane Snowmobile Rentals
Dorset (Muskoka): Mountain Trout House Marina
Dwight (Muskoka): Back Country Tours (also offers guided tours)
Embrun (Eastern Ontario): Loiselle Sports
Gravenhurst: Campbell’s Landing
Haileybury: Work ‘n Play
Haliburton: Back Country Tours, Haliburton Forest & Wild Life Reserve
King City (Central Ontario): Ontario Powersports Rentals Ltd.
Kirkland Lake (Northeastern Ontario): Speedy Snowmobile Rentals
Minden: Harper Powersports
Orillia (Central Ontario): Cottage Country Tours (also offers guided tours)
Penetanguishene (Central Ontario): Diverse Rentals (also offers guided tours)
Perth (Eastern Ontario): Outdoor Adventure Rental
Port Carling: SWS Rentals
Port Sydney (Muskoka): Muskoka Wild (also offers guided tours)
Sault Ste Marie: River City Motorsports
Shanty Bay: Smart Adventure Programs
South River: Eagle Lake Narrows Country Store (also offers guided tours)
Temiskaming Shores: Temiskaming Powersports
Wawa (Algoma Country): Jones Power Sports
Lodgings Who Offer Snowmobile Packages & Ontario Snowmobile Rentals
Dubreuilville (Algoma Country): Magpie Relay
Durham (Midwestern Ontario): Homestead Resort
Haliburton: Bonnie View Inn, Pinestone Resort, Sandy Lake Resort
Huntsville (Muskoka): Hidden Valley Resort, Deerhurst Resort
Searchmont (Algoma Country): Mountainview Lodge
Shanty Bay: Horseshoe Resort
Muskoka: JW Marriot—The Rosseau Resort & Spa
Port Stanton (Muskoka): Bayview Wildwood Resort
Whitney (Algonquin Park): Couples Resort