I have not been able to find a legal vendor still selling the product. Drivethrurpg, Warehouse23, Paizo, Ebay, and Half Price Books don't even list the product, and Noble Knight still lists several packages but they are "out of stock". (I think I saw one Metacreator/Cthulhu package listed as a used product somewhere, but that is not helpful for an Ars Magica player.)
Sorry to have only bad news. Your best bet--and it's a longshot--is to find the license holders and send them money for formal permission, and then get a copy from one of us who have it. (I'd happily send you a copy if you have proof that the vendor gave written permission.) Or if you are really persuasive, talk them into open it up as free abandonware, with maybe a donation link if you are particularly grateful.
But that's going to require rolling several ones in a row on your stress die. Good luck. 