I'm sure you used to be able to get these - are they still available?

It needs to be something entirely dumb - no control sequences or anything - for transparent retrofitting to an existing device.

The connections are 3-wire RS232; Tx, Rx, and Ground.

                         Switch
                       +--------+
Serial         3       |        |              Device
Source 1 <=====/======>+        |             +------+
                       |        |      3      |      |
                       |        +<=====/=====>+      |
Serial         3       |        |             |      |
Source 2 <=====/======>+        |             +------+
                       |        |
                       +--------+

The idea is that, as soon as the Switch detects any serial data activity coming from one of the sources, it switches that (Tx & Rx) through to the Device.

Obviously this is quite crude and relies upon both Sources not transmitting simultaneously, etc - but I'm sure such things used to be available.

I can find a few "intelligent" ones that provide buffering and stuff, but require the Device and/or Sources to send control strings to select the path.

To give an idea, this one sounded hopeful, but is no longer available:
http://www.wut.de/e-85215-ww-daus-000.php

:-(

No, it wouldn't be too hard to do with a flip-flop and some controllable transceivers...