The GBM actually lacks GB/GBC hardware so there is little point in tracking down GBC flash cards.
Supercard miniSD..... it might work but supercards were never great for GBA use so I can not be sure it will work (a quick search looks pretty positive and if I recall it did have an old but built in version- get the real thing though) but as you asked http://www.dwedit.org/gba/goombacolor.php
Just a heads up, you might want to look up some information about using Goomba/Goomba Colour on the EZ-Flash. There are a few issues with saves becoming corrupted/disapearing on the EZ-Flash IV. But as its already been stated, no other way to do it unfortunately. Best bet would be an SP if goomba doesn't work, or even a GBC.
Hmm ok, I heard from some people a while back that the method mentioned above still caused corrupted saves for some games due to the size of the save state being more than 32kb or something. But if that works then it is always good!
1. Create a Goomba Color compilation like normal, using the latest 03-31-2008 version from http://www.dwedit.org/gba/goombacolor.php and make sure it works when you open it with VBA
2. Run it through the Supercard patcher. In the patcher, double-click and select only the first saver patch. Output it to your hard drive and copy it over to your card.
3. Launch the compilation and play your games. Save in-game, press L+R to finalize the save within Goomba (only really needed for 32 KB save files but it's a good practice to get into), and then use the Quick Power Cycle method of saving. You might also be able to enable the button-combination of saving instead of QPC but I have not tested this, as I normally use my Flash2Advance Ultra for playing Goomba Color, not my Supercard.
Does the Quick Power Cycle method work for you at all? Save in-game, power off, QUICKLY power on in GBA mode, press Right to get to the Saver tab, navigate to the *.sav file on your card for the Goomba compilation, and press A twice.