Tractor Supply usually carries it, and other feed stores. The fluids/electrolites would be my main focus. You can also mix some raw egg yolk in water and give that for a bit of nutrition, but fluids come first.
I'm very sorry, it sounds like she's failing. I would try to give her fluids every 20 minutes, as much as you are able to get her to take. If you have electrolites then alternate electrolite water with plain fresh water. If she's dehydrated then that will take down her quickly. Use the...
If she's constipated then she is probably not eating and drinking well, which will make her weak. The nutri drench may help give her a boost, and may help with appetite. If everything starts moving better then she will start eating and drinking better. Chicks can paste up due to being to warm...
I would not worry about the dirt sticking to the ointment. The ointment will keep it moist and help keep droppings from sticking and drying on there. I would reapply as needed. If it is a prolapse, then it has to be kept moist or the tissue could become necrotic. The dirt is unlikely to do...
This may just be a weak chick and there may be other issues that are not obvious. I would gently clean the vent and apply some vaseline or plain neosporin to keep it moist. Take some coconut oil and freeze or refrigerate until firm, break into tiny pieces and feed it to the chick, if it's...