I'm glad you want to change vets. Because my first concern would be what was the shot and what are the meds for? In the case of constipation I would expect the vet to do an x ray to see what's going on and then to give an enema, done without anesthesia I believe. There is another procedure that is done under anesthesia for a more serious impaction if the enema doesn't work.
An excellent source of information for natural cat care is a book called "The New Natural Cat" by Anitra Frazier.
It seems to be over a week now that your cat first showed symptoms. Is he ok?
I have found Bach flower essences to be very helpful for both your cat and you! You need to remain calm, because your kitty picks up on your emotions. Think soothing thoughts to relax your cat's body. Massage him and let him know he is safe.
Anitra Frazier says on page 296:
"Constipated cats tend to hold tension in their bodies and sceret worries in their mind."
Get them at any health food store. There are essences sold at some pet stores but I would get the ones for humans so that you can be sure you are both getting the good stuff. BACH is the brand to get.
Here is how I give to my cats. I pet them and then squeeze the dropper a drop or two on the top of their head and rub into the skin. It is difficult to give under the tongue because they don't like the alcohol base taste.
For YOU, you put a few drops under your tongue...or if you have compromised the sanitary quality of the dropper tip, you can also rub a few drops into your forehead or temples.
I wish I could recommend a good holistic vet but I need to find one myself. However, you can still find a vet who is not holistic but would still know how to handle this situation properly. It is common. Call around and ask questions!!
Let the vet know what you want. Definitely read Anitra's book. You will become a pro at talking to vets and getting the kind of treatment you want...and at doing a lot on your own! Best to you!