I think is rather troublesome to use flashcards because for once, is not as you may think it is, is not as you can just put an R4 and then use a .cia, for this method you need a NDS/NDSLite so you need to:
1 - Get the savefile from your VC game
2- Convert it to an emulator/flashcard-able savefile
3- Transfer said save file to your gba flashcard of choice
4 - Set up both the flashcard and a Gen 4 game on your DS
5 - Do the whole transfer process from there
If you ask me the quicker method is to just use PkHex, but if you're insistent in the method with flashcards, then
http://www.nds-card.com/
May work, or just getting an EZ-Flash and a slot1 flashcard for your DS'll be enough for what you're looking, this tool may work too
https://gbatemp.net/threads/pre-release-agbsave9-tool-to-extract-agb_firm-saves.410684/
That's about the Info I know