My sons exwife says I can only see my granddaughter on my sons weekends. Can I get visitation rights as the grandmother?

This depends on where you live. Grandparents rights have been gaining ysit or ground in recent years. Almost all states now have statutes dealing with it. However, those statutes vary widely. You might try the links under your post to get more info. But you need to tread carefully here. If you do take the mother to court and win, you can imagine that any hard feelings that now exist will be intensified. I would try trying to win her over, offer to babysit or help out in other ways, before you take her to court.