We were there over Fathers Day weekend and Arlene had just passed throuh and dumped a ton of rain. The best place to go into the North Maine Woods is at the 20 Mile Check Point. There is gas and diesel in the woods at a couple of the organized campsites but as you would expect it is kinda pricey and some are cash only. There are plenty of free campsites that you can camp at all through the woods and organized campgrounds with a small store ,showers,restrooms , the whole nine yards. http://seboomookwildernesscampground.com/page1.htm
The only thing that you really need to know about at this time of year is the flying insects. Insect repellant is an absolute must , if you don't have it they will pick you up and carry you away. It is enough to drive you completely insane. Another site that can help is this one , it gives you fee's and other info, http://www.northmainewoods.org/