I'm putting together a pistol.  Can anyone recommend a good buffer tube set up?

Thanks

Any CAR length tube, buffer, and spring.  Yes, they are legal to use just as they are.
I used the ACE ARFX entry buffer tube but if you use this tube you will need their incredibly pricey, but nice receiver end plate ($35).  The standard CAR tube will work just as well and use the standard REP and castle nut.  Also by using the standard CAR tube if you want to turn your pistol into a rifle for a while just slap on a 16 inch or longer upper and throw the butt stock from the CAR or M4 on and you're good to go.  IF you do not also own a rifle upper, I would not advise owning the stock attachment for the CAR tube because if you did you would have all that was needed to make an illegal SBR, and since it could not be assembled in any other way, you would have little defense.  

Thanks for the info.  I saw a short tube with a sling attachement on the end that looked interesting.  Olympic sell a pistol with not tube, just a flat cap.  Any idea how they do that?

They do not use a recoil buffer/spring/tube in their OA-93 like most ARs do, but a completely different system.  From what I understand it works pretty well too but I have no first-hand experience with them.

If that short tube you saw is like the Model 1 pistol tube, you will more than likely be very sorry if you go that route.  On almost every page of the build it yourself and the general AR discussion boards you will see posts about pistol builds.  Very few have had any luck at all with the Model 1 tube with most that started with them abandoning them for reliable CAR tubes.