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- Modded
--'07 BMW M5/6.:Indianapolis Red
'91 BMW 850i J Spec-ST Tuning Kit- Lowered-Wokke Chips- Custom Exhaust- 18" MK1's-KN Filters+ M Steering Wheel- Stereo Upgrade +++
'88 BMW 635CSi Lowered
'81 MB 500 SEC Koenig WILD RIDE