I did some serious searching a few years back, and was only able to find mammoths, buffalo and zebras in the “cheap” category... Everything else I found (antelope, cave bears, giant sloth) was undersized or oversized for 28mm. I ended up spending the cash to get metal figs not made anywhere in toy market (megalocerus, woolly rhino, etc).
I have a post in the pulp section which covers alot of details on the figures.
http://leadadventureforum.com/index.php?topic=106088.0Metal or resin 28mm scale:
Safari Ltd does a great adult mammoth.
Stock figure in upper left. I customized the others by repositioning the heads and trunks.
Other juvenile and infant mammoths from a variety of toy or ornament sources.
Before and after for safari limited mammoth, converted to a calf:
Buffalo and horses (repainted zebras) are plastic toys from mojo miniatures.