Are there any other fenders (like 380SE etc) that would fit perfectly on a 126 1981 300SD?

I think fenders from any 126 sedan will fit. The lower trim of an 86-91 won't visually match the rest of your 81 lower trim.

I'm not sure what was available in Canada but just about anything that has 4 doors and an S in its name from 81-91 is a viable donor.

300SD/L, 300SE/L, 350SD/L, 380SE/L, 420SE/L, 500SE/L, 560SE/L. Some versions were not sold by MB in the US. Maybe they were in Canada.

Are you an S13 or S14 guy?

Sixto
91 300SE
87 300SDL
83 300SD

K&K or Potomac German Auto, both of whom sell fenders, etc., and would have the info as to what would interchange.