I don't think you need the pulsar. At least not in Elite.
I think any small planet without atmosphere and without a significant magnetic field that orbits in the Van Allen belt of a body with a strong magnetic field could be suitable to harvest antimatter in the form of positrons.
Nebulae are 'full' of ionized hydrogen. That's basically free single protons. These can be used to produce antiprotons. Mix the anti protons with positrons and you have antihydrogen.
That's how they do it in CERN.
The problem is storage. I guess that's where the Meta-Alloy comes in handy.