All I know of are as old, rare and (most often) as expensive as the IBM/Lexmark M15.
Cherry G80-5000 (Cherry MX Brown), Northgate Omnikey Evolution (white ALPS), REVEAL (white ALPS) and
Marquardt ergo (tactile Marquardt switches) have a standard layout.
The Kinesis Advantage (Cherry MX Brown) and Maltron (Cherry MX Brown or Black?) have Cherry MX Brown switches, but very unusual layouts.
Then there is the µTron, which has standard QWERTY layout except that the staggering and layout of the non-alphanumeric keys is unusual. It has Topre switches.
A lot has been written and discussed on this board already about all the keyboards that I have mentioned above.
My best tip for buying would be to set up on eBay a query notification (or whatever it is called these days) that will email you when new items show up that has certain keywords. Then, when you see a split ergonomic keyboard that you don't know about, go search here on Geekhack to read more about it.