Ways to get a second monster for breeding

I think there should be another way to get a second monster like crystal, shadow, or escarglow so that they can be breed for higher grades. Maybe once you have got the first one, an option to buy a scroll and synth a second one should occur. Because after completing all the vet modes, I got a crystal but it was e rank and I have no way of increasing to s rank

Just synthesis pure S monsters only or keep them for your encyclopedia only.
Why do you want rare monsters?