Should you buy your ex a Father's Day present? My suggestion is no, you should not buy a present for your ex '- especially not "for good times' sake." He has remarried, and his new wife is now his primary partner. A good-times'-sake Father's Day gift might look like your attempt to drive a wedge between them. You should, however, make sure that Dad has a present from the boys. Continuing these rituals is a good way to demonstrate positive behavior for your children. This reinforces what you've probably told them: "Even though Mom and Dad are no longer together, we respect each other and we both love you very much." As you pick out the present, you can confidently discuss their father's likes and dislikes, and since he has a new partner, it's also a good time to reinforce her influence in the mix.
For example, you might say, "Hey, how about this fishing pole for Dad? You know how he loves to fish. And we can buy this great lure for Michelle [Dad's new wife], so she can help him when he's at the lake!"
What does this gesture say to the kids? It says "Dad and I both love you. We respect each other even though we are divorced." This small act takes the kids out of the middle, and they won't feel as if they must choose between Mom and Dad.
When parents are no longer together, their relationship toward each other changes, but their relationship as parents does not. So, in the future, if you're thinking about giving an ex a present '- for Father's Day, a birthday, Christmas, anytime '- my answer will always be the same. If the present is from the kids, then yes. If the present is from you, for good times' sake, no.