The device you are looking for is Lionel part number 610-5906-001, "Sound Activation Button." They may still be ordered from Lionel Customer Service under "Order Lionel Replacement Parts." Look for item #9 on the page that comes up when you key-in the number above. $13.00 each, you may want two depending on what transformer you use.
The instructions for hooking up either one or two of these are easily found in the Owner's Manual for any modern loco with RailSounds.* The devices essentially replace the old 167's and will work with postwar equipment, although obviously you need a loco/tender with both a whistle and a bell for optimum performance. (There have been mixed reports about how well these devices work with old pre- and post-war air whistles, but my results have been good. They seem to work great with modern air whistles, as well as RailSounds, etc.
Again go to www.Lionel.com , Customer Service and search under Owner's Manuals. For example, look up the manual number 71-1100-250, page 12. Hook up is simple. Whether you use the existing whistle control button on,say, a postwar ZW and add just one 5906 to ring a bell; or add two 5906 buttons to use with both whistle/horn and bell. Remember that you can't sound both whistle/horn and bell at the same time.*
If you have a very old transformer with no whistle button, you just need one 5906 to get a whistle controller. You'll need two to get both whistle and bell. You cannot "sound" a device you don't have, so your train must actually have a whistle/horn or bell, or both.
Note: there is a device similar but certainly not equal to the 5906 with a different number. It is for an arcane Lionel transformer that I have never seen. You want the 5906.
*Here's what I do: With my postwar ZW, which has good-working rectifier diodes and clean contacts, I use the built-in controller to blow a postwar whistle or horn.
When I run a modern Railsounds loco/tender or boxcar, however, I wire up two 5906 buttons as shown in the manual, and never use the button built-in to the old ZW. In this situation, I could "get away" with just one 5906 but I choose not to. My thinking is "postwar with postwar; modern with modern."
I protect my transformers with fuses or circuit breakers; and the modern electronics with TVS's.