Iirc, all batteries are put in designation categories.
Go to a local NApa or something and get them to look up a battery, they will tell you what the disgnation is which fits. There is nothing special about an M5 battery, you just need one that has the + and - on the right side (if it even matters) and it fits in the tray and provide the output which is necessary for the V8.
From my experience, my Range rover, my e34 M5 and current M5 use pretty much the same battery.
I bought a battery for my e34 from Eurostyle (it was used but only 1.5 years old for cheap) and when I needed to get one for my RR I got one from Napa for like 140 bucks or something where the dealer wanted 3-400$.
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--'00 BMW ///M5 .:Jet BLK/BLK:. Individual - Style 65 Wheels; 9.5" Square Set up;275 Tires;10mm/3mm Spacers; Black Kidneys;Muffler Delete; 3.5" Dual Tips; Beast Power; "Dinan" Ducts; E60 SSK; BC Coils
--'00 BMW ///M5 .:Avus Blue/Grey/Black:.
'88 BMW 635CSi Blue/Tan Sport
'81 MB 500 SEC Euro Koenig
'91 Alfa Romer 164S- Red/Black/ Recaro interior