Okay...so I disagree a bit with others here. I think that there are a lot of men out there who would be willing to watch their wife with another woman, even without touching. However, going forward with something so that your wife can have her fantasy is dangerous. If she isn't ready or still has feelings of insecurity then now really isn't the time.
I would suggest you talk more with her about it. Tell her how you feel and listen to how she feels. If it is something she really wants to do (not all of us want to do all of our fantasies) then have her decide the best way to go forward with it (including talking to any friends she might be interested with). A club is a bad idea. The chances of finding a woman at a club are pretty slim and if you aren't interested in doing more then just having some one on one for your wife then it could be hard to deal with everything else there.
However, whatever you do, before you do it....Talk more about it. If she still isn't over any insecurity or jealousy issues then there will be problems.
Good Luck,
Dani