Varney's has a very good rep but it looks like it's for sale and appointment only. http://www.varneysclaysports.com/hours.html
This is a list from Gunners of Maine. https://www.gunownersofmaine.org/ranges#:~:text=Rangeley%20Region%20Skeet%20%26%20Trap%20Association%20%28Oquossoc%29%3A%20http%3A%2F%2Fwww.rrsta.org%2F,Gloucester%29%3A%20https%3A%2F%2Fwww.royalriverrodandgunclub.org%2Findex.html%20S%26S%20Weapons%2C%20LLC%20%28East%20Millinocket%29%3A%20https%3A%2F%2Fwww.facebook.com%2Fssweapons%2F
Blue Hill is near Bar Harbor. SSW of Ellsworth. 5-stand only. https://www.bluehillrpc.org/
Arnold Trail in Sydney (between Waterville and Augusta has skeet, trap, 5-stand, and sporting clays according to their website. http://www.arnoldtrail.com/wp/
L.L. Bean does 5-stand in Freeport
There's a bunch.