Board Thread:Warhammer 40k General Discussion/@comment-25149066-20140708202914/@comment-25177957-20140714222301

"His masters required more Space Marines, more than could be provided by the harvest of the gene-seed from those already serving their unholy purpose. Fabrikus had spent the centuries experimenting with the other races available to him, but the gene-seed refused to take, or else it produced mutations that were unhelpful. Therefore, he had decided to return to take up his earlier role as an Apothecary and harvest the Progenoid Glands from a more pure source, unaffected by the energies of the Warp -- that of captured Space Marines loyal to the Emperor. In this way, these new measures would help ensure his success in the creation of new types of Astartes loyal to the Ruinous Powers of the Warp and unstoppable in battle."

Fabrikus was an apothecary of the World Eaters, so this is obviously how they're getting gene seed and new recruits.