I need to make Pasta Zn Oleosa, but for this I need oil acid which is out of trade... How do I best solve it?

To begin with, I must point out the inappropriate use of boric acid. Boric acid is still only allowed as a buffer in eye preparations. One could use here a better antiseptic such as chlorhexidinediacetate.  

Oil acid is added for the formation of Zn oleate (W/O emulsifier) that performs here the function of humidifier of ZnO, a hydrophilic substance. The   is no longer available and there is very little for substitution. I see only one possibility and this is Arlacel C. For the rest, the ointment-paste can not give any problems. If you have an ointment grinder, it is appropriate to feed the paste through the ointment mill to break all the ZnO agglommerates. Prepare best 10% excess.