Analogue Gauges
For "steam" gauges there's SimKits that sells kits that utilize radio control "servos" to move the gauge hands.
Other option is http://www.sgsim.com/ .. check it ! Price indication are € 200 per master gauge and € 100 per slave. Each master can have up to 4 slave gauges attached.
Yet another new online source for analogue gauges is http://www.flightillusion.com They offer gauges that are only 1 inch or 25mm deep !
it may be worth looking on ebay for analog instruments. Most of them would need to be retrofitted with servos to make them work for your sim.
Build them yourself.
Mike from mikesflightdeck.com has written a book on building simulated instruments. Its due in a few weeks. Check http://www.mikesflightdeckbooks.com for details.
build them from scratch, similar to the Simkits instruments, as Stephen has done for his C172 cockpit: http://cockpit.varxec.net/stephen/
Also, just like people use monitors behind panels to simulate the "glass cockpit", it is just as possible to do round holes on the panel and place conventional Flight simulator panel with round gauges behind so that they align with the holes.
Other than that you can find multiple (also freeware) analogue gauges on external exe file so you don't need to take an extra bite out of your FPS in Flight Sim.