I bought my mammoth from the Toy Soldier HQ. Their website is atrocious—it looks like it was designed by a kid back in the early '90s—BUT they have alot of old Marx models unavailable elsewhere. The Mammoth is the perfect size for mixing with 28mm cavemen. The Marx Megatherium also works well.
I love this pose, but I must say the tusks were sculpted way too high on the face. Tusks are teeth and should be coming out of the mouth! I mounted the mammoth on a 60mm plastic base. To paint the tusks I followed a GamesWorkshop tutorial on painting their LOTR Mumakil model. Interesting note: The words, "Wolly Mammoth, 23' Long" are moulded on his belly.
I am now looking to buy some cavemen. I think the Vendel Miniatures Hillmen will make excellent cavemen (if I remove the shields). Copplestone Castings, Dazed Miniatures, and Jeff Valient Studios offer very nice cavemen too. It's the archer models that draw me to Vendel. The others don't offer cavemen archers.