What kind of aluminium grade for a heated 3D printer bed? I'm replacing my standard MK3 Aluminium printing bed with a custom sized  bed paired with a silicone heater. I came across a few on Amazon and they all have grades such as 6061, 3003, 5052 etc. I know these mean the composition of the Al alloy and their end purpose but I'm not sure what they mean in terms of being used for a 3D printer. Can anyone here advise me on this?
Various grades and alloys of aluminum will have characteristics related to ease of welding, resistance to corrosion, malleability, and other aspects. For a heated bed on a 3D printer, you'd really want to have something resistant to warping and something that can be assured to be planar across the surface, that is, flat.

The terminology you'd want to use for your search is "MIC 6 Cast Aluminum Tooling Plate" and the results are many.

From a rather comprehensive web site:

Flatness tolerance is maintained within .015" on 1/4"-5/8" thickness and .005" on 3/4"- 4" thickness. Thermal Cycling can be performed up to 800° F  under controlled conditions.

You would want to confirm from the seller that the surface has been prepared, as some sites appear to sell un-finished tooling plate, but I've not been able to clarify that. Most appear to provide either no specifications regarding flatness or give a figure such as that above.