Where are you located.....I.E. USA, Canada, Europe....where?
Where you are will probably narrow your choices in aftermarket fins.
Select fins (from France) seem to get good reports.
Gorge Fin Co., True Ames Fins, and Tangent fins are good here in the US
I think if you could find a 42-46 cm slalom fin would work pretty well with an
8.5 m2 Gaastra Matrix.
Look around and see if your local shops have any fins in this size range.
Even a good condition used 42-46 cm would be quite good.
You are looking for a fairly vertical planform fin something like this:
Or the Freeride Swift series would work as well.
If you can find a photo on one of the fin mfg.'s websites,
send me a link and I can suggest whether it would be appropriate
for your board and rig.
Hope this helps,