TwinCam

I went to put eyelets from a CTEK trickle charger on the battery posts on my 2010 E 550 and found the bolt extremely tight. The square head on the back of the bolt is very difficult to grasp tightly with a pliers and I can barely turn the nut. The clamp is configured in a way that it is difficult to get a pliers on the bolt head. I would like to replace the bolt with a new one when I get it off, any ideas where to get new bolts and the best way to remove the current bolts? The nut remains very tight and difficult to turn even as I started to loosen it...I don't think it is stripped, just very tight. Thanks for any assistance. TwinCam