Why get an aftermarket mechanical boost gauge? The factory gauge is just your computers best guess as to what boost levels are.  It can be off significantly from what your engines vac level actually is.     Where can I get an aftermarket gauge? Aftermarket gauges are available from many suppliers and companies (too many to list).  Any 2 1/16" boost gauge will fit in the stock gauges location as well as aftermarket gauge pods such as the Lotek pod.  You can buy them through any parts supplier

1st gen valve "crushing".   If you have a 1st gen valve assembly you may consider crushing it.  If you run higher boost levels then stock your valve may open too soon if you do not crush it.  Your idle may also have a lower HG than the valve does.  This may cause the valve to open while you are at idle.  It is best to have an aftermarked boost gauge hooked up so that you know what your actual idle setting is.  The stock boost gauge is just the ECU's best guess as to what your levels are.

Know anyone who makes 'em, eh?

The factory code is T87/B5 and the PPG code is 16575 Thanks to chuck641 for the info.