OMG, that stinks. However I guess the concern of non native being grown is probably why . I would go to your local nursery or a home depot if you have them there and see what they may have that you could grow and feed. I am sure you should be able to find lots of different seed that they will eat. Even if you can only find a grass or clover. Good luck.
Why would they be in your seeds? I really don't think you have to worry about it being in the seeds. If by chance it were any in the seeds, I would think it would be so minimal that it wouldn't matter anyway. If it were me, I wouldn't worry and buy some seeds and plant them for your torts. They will love it.