They're not too hard to find. There are probably 2 or more listed on Ebay at any given time. Price is a whole nother issue, though. There are several different parts that you need, and rarely can you find them all in one package. The only complete sets I've seen listed go for $2,800+ (
http://cgi.ebay.com/ebaymotors/Restored-Ebody-Rallye-Dash-Kit-1970-71-Challenger-Cuda_W0QQcmdZViewItemQQcategoryZ140720QQihZ005QQitemZ150103597553), but if you are patient you can pick up all the individual components for under $1,000. I'm in the process of doing this right now as a matter of fact, and have everything I need for under $700. The only issue will be whether or not I will have to have any of the gauges repaired, but even worst case I will be under $1,000. The best way to search on Ebay is to just do a search exactly like this:
dash (rally, rallye, ralley) , and
switch (rally, rallye, ralley) . The different spellings are because a lot of times the listings will be spelled different. You will get a lot of results, but this is about the best way I found to cover all the bases. One other thing to keep in mind is that the dashes were different for 70-71, and 72-74. They will interchange in the cars, but the reset knobs were in different locations for the correct period. I hope this helps.
Here's a list of the key items:
Gauge Cluster with bezel
Switch panel (3 or 4 switch) with bezel
Rallye dash heater control panel
Rallye dash wiring harness (standard harness can be modified with the right parts)
Reset knobs (typically have to be bought seperate)
New woodgrain overlay
That's about all I can think of at the moment. If I missed anything else feel free to chime in.
