I'm sorry to say that WTB threads aren't allowed on this forum. However, since you're looking for something faster then a SS7 board WITH ISA, you have multiple options (most of these are actually quite easy to obtain, and probably very cheap also 😉 ).
The easiest option would be to get a s370 board with the VIA chipset. Most common are the s370 boards with the 694X chipset. This is basically the same as the KT133A, but made for Pentium 3's.
The 694X supports Coppermines (getting a 1Ghz CPU should be a piece of cake) and the 694T chipset also supports Tualatin.
The fastest Tualatin around is the P3s-1400. 1.4Ghz, 133Mhz FSB and a whopping 512KB L2 cache. This CPU can be bought for under €10 NOS.
On top of that, they run MUCH cooler then the Thunderbirds or XP's and also support SSE, which the Thunderbirds do not.
Tualatin boards with ISA slots aren't very common though, but the same goes for KT133A boards with ISA slots.

I wouldn't get a KX133 board, unless you have a thing for Slot A.

Edit: And welcome to Vogons! 😉

My tip is to build 2 machines.

One 486 or SS7 system with ISA slots for older speed sensitive games like Wing Commander, Heart of China or Test Drive 3

And another one, PCI only for Windows 98SE. Something S370 based, cheap and easy to source!