I would start with a special needs parent support group, these usually happen on a weekday night, so maybe dad could watch then then. Also, why isn't he taking them in the evening after school so that after YOU get off work, you can have some grown up time? I used to have the same problem, went to therapy, and the first thing she made me do was schedule alone time, and couples time with my hubby. I realize you don't need that, but, you do need alone time. You need to have adult conversations, you need to be able to get your nails or hair done, and let me tell you, for $30 is amazing what a mani-pedi can do to make you feel so much better!
I am in Portland, so I can't give you a great therapist, but I have seen several good ones on Craigslist. Also, where is your respite care from you son's Support Disability Services care coordinator? Do you have one? If not, why not? You can't do this all alone all the time, and I do understand how hard it is with a special needs child. But, at the same time, you need help. A counselor is a great idea! I have a feeling s/he will help you make a plan on how to get what you need. The first thing is to get dad more involved, and get him to take the kids when he is off. You should not have them 7 days a week when he is in the picture.
If your son is not ok with staying the night at dad's because it breaks his routine or whatever reason, then dad needs to bring them to you house, watch them, do night time/baths, dinner, etc. and put them to bed. It does not matter if you are there or not. He is responsible for them until you are reading to take over at whatever time YOU want, or you want to go to bed, but not one minute sooner!
Will your insurance pay for counseling? I sure hope so because it can get expensive, but if no one can give you a good recommendation, the you can find one who takes your insurance and then see what they work with, some specialize in parents of special needs kids, and some specialize in single moms or family counseling, while others focus on other things.
And, if the first one doesn't work for you for whatever reason, please do not give up! My hubby is on his 6th one and it took me a couple before I found one that I could work with well.
Finally, realize mama that you are not alone! I went to my first support group last week, and it was so refreshing to know that I/we are not the only ones who have the same fears and concerns about our kiddos. Other people do too! and, I got some good tips from a dad who has 2 special kids.
Good luck, and remember this will not last forever, like you said, next week will probably be good, but that doesn't mean you need to forget about the bad times if you are having a good week when your therapy appointment is, you still need to go because the struggles will return.
ETA: I found this place in Plano on Craigslist if that helps: http://www.nathanielsmithcounselor.com/.