Look for the wild milkweed.
Contact a local garden club, maybe one of the members has milkweed.
Contact a local nature club and maybe they know of some resources for milkweed.
Botanical garden in your area?
Can you buy anymore? Maybe a local garden center has more milkweed that hasn't been sprayed with pesticides?
Just some thoughts and maybe one of these groups in your area could direct you to some milkweed.
I'm having same problem with my milkweed.
The yellow aphid, some other kind of small bug.
Rust on bottom of leaves and then the leaves yellow and fall off.
Black mold on tops of leaves.
Most of my milkweed was chopped back and it looks healthier now. The trimming of milkweed seemed to make it grow again.
Also, when you get milkweed for your cats. Do you just give them leaves?
Or do you cut a stem for them?
I found real good luck in just sticking the "bared" stems that cats ate into water. Within a couple of weeks these cuttings grew roots and started growing leaves again. I stuck cuttings in ground and they are making some nice younger leaves now. These cuttings actually saved me, since it gave me a new generation of milkweed leaves to use. And now some of the larger plants have recovered.
Saving one egg, cat and butterfly at a time