If you want to use a downloaded ROM you need a slot 1 card and a slot 2 card. You can then get the slot 2 card to act as the required memory expansion. Besides that you can buy it.
You need to download the rom, it can be found on the main RomUlation site, and then you need to patch it to work with your slot-2. AFAIK there are only patches for SuperCard, M3 and EZF 3-in-1. there may be one for G6 but I haven't really checked.
Sucks really. The thing that really sucks is the fact that my Supercard only came with a black casing. I was thinking of swapping my brother back for my White one...