Get them almost anywhere. Sakari probably can order them other wise search around. Autometer makes some, defi, etc. http://www.egauges.com/
I had an Omori 45mm mechanical boost gauge on my '03, mounted in the stock OEM cup. It functioned great, no problems, easy install. +1
I use the Autometer Sport Comps b/c they're black faced w/ silver trim that matches the o.e. stuff. Plus they are not expensive.
I use these too, but the are cheap and not great quality. My EGT is going out, and my Boost gauge is off by a couple PSI. They do match the dash almost perfectly, but next time, would definitely spend the extra couple bills for the Defis or something better quality.