Which computer is going to house the modem?
PCI modems are easily found, but many of the ones sold today are
so-called "Winmodems" that require the software driver to do all the
hard work. Most of them work well enough, but you'll want at least a
Pentium 233 or higher for the best result.
If you want a "real" modem or need something that plugs into your
RS/6000 machine, an external serial modem is probably the ticket.
Trouble is, some of those are now Winmodems as well.
I make a habit of buying up lots of external modems secondhand...they
are nice to have to hand out to customers when their internal modem has
died. All that's usually required is a simple change of the serial
cable to one that works with their computer.