This
is a tough question. I'd probably give up on any idea of getting Grandpa's ashes - your dad probably did something with them by now.
While I agree with the others that what you do is for YOU at this point, I also would feel the need to honor Grandma's wishes.
Hmmm is there any way to make something with the ashes? Some sort of keepsake? (Do they do that sort of thing?)
If the ashes could be put into a lovely statue that would be something passed on to your children & grandchildren.
Other than a vault at a cemetary I can't think of anything else.
No matter what you do, I'm sure your grandma would understand your dillema and be fine with whatever you decide to do.