If you want something that works out of the box, the suggestion above is good.
Zalman also had this really strange beast, the CNPS6000-Cu, that could be used with (almost) any fan you like. Might be hard to find today though.
Note that socket A coolers are compatible with S370 as well. There were a few quite beefy models with 80mm fan for those power hungry late Athlon XP:s.
Otherwise, if you have access to a 3D printer, you could just equip any old heatsink with a shroud for any fan size you like. Done it myself a few times with fantastic results. Designing it is a great first 3D modeling project. I don't have the energy to tear down a retro rig to snap photos right now, but here's a screenshot of my 3D model:
The attachment shroud.jpg is no longer available
If you happen to find a heatsink that's 54x54 mm I could of course upload my model for you too.