You need to find a power supply that is adjustable, most have a pot with small voltage range, so if you can find a switching supply that gives 13.8v you should be able to tweak it to 15v. Be sure that it will supply the current required as if it is under-rated then excessive current draw will not only result in a drop in output voltage but also overheating and ultimate failure of your supply.

To figure what you need then add the current required by all the peltiers and multiply it by the desired voltage to give you the total power requirement. Now add at least another 50 watts for headroom/safety. Now you know the size of psu.

You failed to mention if you were running three 50 watt pelts, if so that's a total of 360 watts (3 x (15v x 8a)) - so that's a 400 watt supply to be on the safe side. However, based on your temps I suspect it's a total of 50 watts peltier cooling, so get a 15v 10 amp supply - but it's not going to help that much as 50 watts is insufficient for cooling your Athlon.