I am not in Maine, but thought I'd give a few suggestions.
Are you members of your local homeschool support groups? Have you called any art museums around you to see what classes they offer and whether they have any homeschooler classes or events, themselves?
One of the more art-minded moms in our homeschool support group hosted an art club for years where she taught the children to draw with charcoal and other mediums, then later to work with Indian ink and other things.
In addition, one of our local art museums gives us a free monthly field trip, complete with docent-guided tours of one exhibit, then a related art activity to go along with it. We have a huge turn-out every month and it's fantastic. They do these sorts of things for schools and were happy to offer it to our homeschool group when we asked.
Have you tried Parks & Rec in your area? Our Parks & Rec offers a weekly homeschool art class. Granted, it's often more crafty than artistic, but it would be nearly perfect for 8 & 9 year olds. If they don't offer any such thing, it can't hurt to ask if they offer after school classes like that, or if they would be willing to start one up. My local community center has totally started classes after I requested them before, with my assurances that I would do my best to advertise them to as much of the local homeschool community as I could.
Have you called any tutoring businesses that might be in your town and see if they offer any homeschool classes at all? We have a math tutoring business in our town that offers homeschool-high school math and homeschool writing classes sometimes. There is a science company in town that does presentations and classes for schools, preschools, at the library, etc., and they also do homeschool science and homeschool art classes, as well.
Additionally, the children's science museum and the area museum of history in a neighboring town holds monthly science and history classes for homeschoolers.
If you are active in your homeschool support group, I suggest posting and asking your group if anyone is willing to head up an interesting class, whether it be a book club or a math club or an art club. You might find that there is someone willing to do a short term class like that, either for free, or perhaps for a small fee. Or you could team up with a few other homeschooling parents and do a short term cllub together, where you each take turns coming up with educational or artistic activities for one get-together and letting the children engage in this learning and fun together.
Good luck in your search!