Safety lever on my P9s fell out on the range, and is lost forever.  I've been trying to find a replacement but I haven't had any luck.  

Anyone know a good source for these parts?  I've tried SARCO, Numerich and HKParts without success.

Did you call HK?

You would have more luck dragging a rolling magnet over the range to find it than locating one on line. These were made in Germany and even the mags were impossible to find in the 80s.
If you could get the blueprint design from H&K a machine shop might be able to make one. Or if you can locate another P9s and a Machinist measure the part he can build it. There never were a lot of them around the pistol was years ahead of its time. Coolest disassembly of any gun ever, and super accurate. Sorry Bro.

Keep an eye on gunbroker for part outs or rough pistols for spare parts. I bought a spare P9S for roughly $400 as a parts gun.

gunpartscorp.com shows the safety lever in stock for $83.75 part #604000

Try ebay as well.

Two new developments:

1. When I took the gun apart to install the new safety, I discovered the old one had fractured into two parts.  I chalk that up to 49 years of metal fatigue.

2. Once installed, the new safety won't engage.  I checked several times and the firing pin is oriented correctly. Function check appears to be okay (except for the safety lever).

Any ideas on what the issue is?

I don't normally use the safety (I load it and decock it) but I'd like to have the gun in correct working condition if someday I decide to sell it.