My ex has become a dead beat dad. I need to modify our custody order... read on for more info. Thanks?

I divorced my ex-husband years ago with agreement to equal time with our kids. We were peaceful and I had (and still have) the boys in mind. They were infants when we separated (They are 10 and 8 now) and the original agreement was joint legal and open everything else. Since the original agreement, he has left the state (for years)... came back.. left again and has been gone for over two years. He filed the original paperwork, which means he spent the money for first paper filing. He's abandoned our children and I want to take full custody. He has not called in months, refuses to give them the time of day, My current husband and I paid round trip to send the boys up for the summer because they live with us but never wanted to deprive the boys of anything...Since they came home, not a word has been said, no phone calls have been returned and they got home over 3 months ago! Shamefully, I had to call the ex's girlfriend's cell phone and text several times to get any type of response. BUT... clearly the care factor on his end is zero.... Like I said previously, I'm wanting to take full custody with child support.... He's been a dead beat dad and I'm tired of it and I'm tired of being "the nice guy". Can anyone point me in the right direction? I need to modify the order but to be honest, I can't afford to pay $300+ for filing fees, after all... school just started and they needed new clothes, shoes, backpacks and school supplies. Got that covered. I hate that I had to even ask this... But.. never the less, your help is greatly appreciated. ~~~~I don't want anything other than cognitive advise, Thank you ~~~~