Plenty are usually available on eBay.
No specific one is going to be "readily available" unless somebody comes across a warehouse with a bunch of NOS boards.
If you really want to go "high-end" you can even get a Pentium IV setup that has AGP and ISA.
Backplane with the required PCI-ISA bridge module can be had for $140 shipped and the SBC can be had for varying high amounts when they show up on eBay.
Backplane: PCIAGP-13S2 BACKPLANE board 4-PCI 7-ISA 1-AGP SLOT PISA-KIT01
SBC: IEI SAGP-865EV-PT SAGP-865EV-PCIAGP Socket-478 800MHz FSB CPU board with AGP 8X
Of course you need an industrial case to mount it in.. but hey, it is pretty much going to be the highest end system with AGP/PCI/ISA you can get.
There are a few other industrial boards out there but getting one with ISA that has DMA support is kinda iffy and usually just as if not more expensive.