Where can I find support in the transition from wife to caregiver?

It depends. Your question sounds as if your spouse has a serious medical illness. If this is the case, you can check with your local hospital or do an online search for support groups in your area based on the specific disease for which you will become the caregiver.
Social worker. Often you can find help by seeing a social worker. Also, there are support groups that meet at local hospitals that can be helpful. Depending on the condition, there are national organizations where you can seek help. Also, simply having time to vent your feelings to a good friend can help. This is a tough thing to go through but by all means do not suffer in silence. Seek help. Good luck.