you can flash the bios your self if you get the blank flash chips that are compatible. just solder them in as the online guides say with a switch to flip between sock and the new chip and use dreamshell to flash any bios file it has a bios flasher tool.
i have a few chips still laying around from when i got mine cheap on ebay a few years ago, just find a sell that sells smd components and look for the flash chip by it's name and not with dreamcast in the search for it, you'll get ripped off that way.
two of my three dreamcast are modded with internal serial sd and ide mod running japanese cake bios and a overclock mod.