What kind of aluminium grade for a heated 3D printer bed?
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.