The grease you need usually comes with the kit.
Bearings should be oiled not greased. There's specialised oils sold by hobby shops and I am not cognisant enough to recommend anything else.
About grease, the diffs need two different kinds of grease (those that come in the kit). One is silicone grease (white, waxy), the other is a thicker, stickier grease. The white one goes on the large diff balls, the other one goes on the thrust bearing/balls. Tamiya calls the latter "antiwear" and this is what they recommend on driveshafts as well.
As for silicone grease you can get the industrial grade (I get the Molykote brand, perhaps there are others) and that's way cheaper and will last you a lifetime so you don't have to skimp when rebuilding diffs.
Ceramic grease is used on gears and again comes with the kits. You can get it in larger tubes from hobby shops.
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