There isn't any local (ie: Thompson MB) canoe outfitters I'm aware of that service Seal River trips.
If you need to rent a canoe and/or gear your best bet would be to do so in Winnipeg.
I've paddled the Seal twice, once starting from Tadoule Lake which you can reach by scheduled flight from Thompson and have your boat and gear shipped in via air cargo. I also paddled the South Seal from South Indian Lake (Leaf Rapids) to Tadoule Lake and then on to the coast.
One very positive thing for anyone considering paddling the Seal is that the train from Churchill to Thompson has just (like this week) resumed service after being non-operational since major flooding last Spring.
You can find my detailed trip report for the South Seal route here:
http://recped.com/seal/The Seal is a great trip, if you have any other questions feel free to ask.