The following comes with the big disclaimer that the kansallispuku (national dress) is not necessarily directly the same as the kansanpuku (folk/traditional dress) that people in a region wore, but are usually historical reconstructions of a specific outfit worn by someone in that region, or sometimes a bit more fantastical imaginings of traditional outfits... but they can serve as a good starting place to look further into things. I'm personally not an expert in folk or national dress, but maybe some of this might help?
Region-wise, Jääski or Ruokolahti's national dresses might be the geographically closest, since it doesn't look like there are any particular to Imatra.
Unfortunately, this PDF is in Finnish, but it has a lot of information about the folk(!) dress in these areas, too (I'm happy to assist with the language if need be!): https://digi.kirjastot.fi/files/original/ded0962cc8eb406cb78688b9433d729b.pdf
Also, finna.fi is a great resource for historical stuff, but you really need to know what to search for. Searching "Imatra" - or as it seems many people have worn Jääski dress in Imatra, using that search as well - among "esine" (objects) or "kuva" (picture) might bring some stuff? After that it's a bit of digging and looking at what kind of clothes get found from different regions.
Again, I'm in no way an expert, but these are resources I've personally looked at in my search for Karelian kansanpuku - I hope this helps!