You know, I had a look at Mansfield's web site, in their FAQ, and I was highly amused by the following exchange, which I will distort a bit:
Q: Do Mansfield toilets use standard parts?
A: We have an extensive supplier network throughout the US and Canada, so you can get parts for our toilets from our dealers!! Yay!!
Nice dodge, because, of course, Mansfields are filled with proprietary parts. Like your seal.
If it were I, I would look up the contact phone number for Mansfield on their web site, and call and ask the question directly to them: "Hey, where do I get this thing in Canada? You say on your web site that it's easy to get parts for your toilets in Canada, and it's more and more sounding like total BS to me. Just sayin'."
Alternatively, I would grudgingly check with these guys: www.mansfieldparts.com Put a slash in and then product 35 then html. (our forum is blocking out the direct link, which I don't think it's intending to do. The whole url should be in the format "www.xxxxx.com/xxxx35.html") They claim to have that part and claim to ship to Canada. Not the greatest solution because you're probably going to pay $15 for the thing with shipping, but maybe that's the easiest solution.
If you are having trouble with my link instructions above, just go to the mansfieldparts.com home page, click on the #10 toilet tank parts list, and scroll down to your seal. It's the leftmost item on the fourth row. At least on my browser. Regardless, it's in there.
I of course have no experience with these guys and have no idea whether they are even legit, but they take paypal, so you can always use paypal and get its guarantee.
Good luck!! And if it's a good experience let us know, because I have seen a number of folks in the past being challenged when trying to get parts in Canada for US toilets of several makes.