You can save seed from your plants. If conditions are right, those diseases can show up regardless, so there is no reason to get new seed (that might also be carrying disease) from somewhere else next year.

If you ferment the seeds for a few days before rinsing and drying them, that will greatly reduce the amount of disease that might be carried over on them. It will also get rid of the gel that surrounds the seeds, which contains a substance that inhibits germination.

I have never tried this, but you can try soaking them for an hour in 91% isopropyl alcohol (available at most drug stores). Dry thoroughly before packing or planting.