You will never fins an ISA slot on any new mainboards today. The limitations implied in this architecture at the development stage in early 80s, when it was the basic feature for PC/XT computers played their fatal role in ISA’s oblivion. Moreover, ISA was not only removed from the PC 2001 specification, but the mainboard manufacturers stopped implementing it at all. Nevertheless, there was a great lot of the controllers developed specifically for ISA slot. And these were not only cheap network cards. Modems, sound and graphics blasters, scanner SCSI controllers and many other things, which you will have to give up as soon as you decide to upgrade you system now.
In order to save you from "suffering severely" for the ISA treasures, Arstech suggested the following. On the company web-site you can right away order a USB-to-ISA converter. It will cost you quite a tangible sum of money. Depending on the implementation (one card or multiple cards connection) you will have to pay from $100 to $170. Of course, you will get a discount if you buy wholesale :-) All converters are intended for USB 1.1, but we don’t think it should be a problem for the guys to develop a USB 2.0 solution for you.