What lift do you plan?
One of the most impressive lifted P38 (Search DAVP38) doesn't have those. If you slightly move the brackets you can gain some length. That's the trick.
But if you really want those, you can have a custom set made for you in any performance part shop
I did this on my '89 RR Classic and assume the same/similar can be done for the P38.
The simplest and easiest solution is to have any competent brake place make some 2"-3" (50mm-75mm) long copper brake lines. Each line has a male and female fitting to suit the P38 brake fittings.
Then you disconnect the current brake line, drill through the spot welds of the brake line support bracket, (on the RR Classic) turn this bracket upside down and bolt it back on to the body. Inverting this bracket should provide 50mm or more drop for the mounting point. You may need to drill more/other holes if you need extra drop of the line (Higher body lift). The P38 manual shows the support brackets already in the lower position than the Classic (inverted when compared to the Classic mounting) so the whole bracket will probably need to be moved down to suit your needs.
Fit the new copper lines between the original brake line and the new mounting point - voila extended brake lines for a FRACTION of the price.
When I did my own 2" body lift on my Classic, I was quoted AU$70-90 EACH for extended brake hoses, a total of AU$210-$270. The copper extensions cost AU$40.