As you didn't post your requirements, here are two suppliers of PWHT equipment, Miller for induction heating and Team for resistive heating. Don't act surprised when you ask the price.

http://www.millerwelds.com/products/induc
tion_heating/

http://www.teamindustrialservices.com/main/index.html

Sorry, I wasn't aware of any portable induction stress machines.  I know of Team, but we want our own resistive machine.  So we don't have to pay team to do it for us.    Thanks

Team sells the resistance PWHT equipment which I think is from the old Cooperheat product line.

I don't know what capacity machine you will need but I would recommend one slightly larger than you anticipate using. If you have any doubt about the size get an electrical engineer involved to resolve any issues.  

The ones we use (contractor owned) are desiel generator 480 3 phase driven.  all the controls are home made mounted in an enclosed trailer.  strip drives and temp controls in a separate room and rest of trailer is for electic pads.

Pretty expensive equipment if you don't use it very often and keep personell trained in use and calibration and set up.

Try Mannings USA. I think they are in NJ and can provide anything you would require...lease or purchase. However for ease of use I would look hard at the Miller Proheat 35 induction unit if you are doing piping.

I use Mannings for power plant work: turbine bolts, turbine-rotor couplings, post-weld heat treatment (piping welds - when needed) and (pre-weld) bending and heating of oddball valve body shapes and configurations.  

They are very, very good.    

CWI88 ... Have you considered vibratory stress relief?  I don't know if it's allowed in your application, but it has been documented to work very well in certain situations.  We have had very good luck with it during and after welding.  Bonal makes the equipment, and has lots of technical data on the process and applications.  I am not positive, but I believe it is an accepted process in certain ASME applications.  I think that Battelle (sp?) Labs in Columbus, OH wrote some unfavorable reports many years ago, but then I seem to remember that they changed their position some years later when Bonal convinced Battelle that they were evaluating an inadequate procedure ... something to do with the resonance frequency of the process had to be adjusted to a certain level to get it to work ... I am a little fuzzy on those details ... too many welding fumes over the years I guess.  The equipment is expensive.  As far as TIS (Cooperheat) we have purchased some used power sources and heating pads from them ... and they are expensive, but they work just fine.  You can also use DC welding power sources too, but you would need to make up your own thermocouple controls if you need to maintain certain PWHT conditions (e.g ramp up, hold temp, ramp down, etc).   However, the welding power sources require a low power heating pad, since you typically can't get the 80v required by the standard pads.  They can make 60v pads that we used with the weld power supply ... but again, we were not trying to control the temperature ..  just trying to heat the part for a different Non-Code welding application.

CW188 Heat Treating Inc. has 3 zone machines that run on 110 or 220 volt with 30 amp service. The run 40 volt ceramic pad heaters. These machines are very reliable for small bore pipe feom 1" to 8" and can reach 1400 F easily. HTI also has larger machines 480 volt 100 amp 80 volt heaters. They make a 6 zone and a 9 zone that are very reliable and not very exspensive.