If you can find them at all, they will be amazingly expensive. I believe Earl ( Earl's Repair Service http://www.carlwalther.com/p5.htm ) had one or two of what is called the " 135mm Long Barrel ". They were not for sale but you might be able to get on the list to be called if he ever does decide to sell one. I forget now if it was $2000 or $5000.
I believe they also have the front sight machined in, and were sold with slides that did not have a front sight.
I sent my P5 barrel to Bill Jarvis to see if he would be interested in making a longer barrel, and he was not interested in doing so, even when I suggested he might be able to sell them for rather considerable prices. I did not try BarSto.