I our lab at the INRA of Versailles, France, we do have a constant rearing of Mamestra and we do sell eggs. The last info I have is that it costs 350€ the 100 egg batches but you will have to talk with Emmanuelle Jacquin-Joly for the exact prices now days. I see your question was posted in May so you may already found the eggs that you need. Sorry to reply so late and wish you good luck with your research
Thank you very much! I managed to get some eggs from Wageningen UR, with whom I am now collaborating, but good to keep in mind - I hadn't thought to ask Emmanuelle!