I don't know of any websites and I'm not anywhere near the same issue. However, I'm the daughter and friend of many women in similar circumstances so I have some ideas.
You could volunteer. This would give you a chance to meet new people, give you something to do with free time, and you could volunteer with children so you can still have young ones in your life regularly.
Ask your daughter if there's anything she could use help with. You're still her mom and just because she's not going to be living with you doesn't mean she won't need you in this new stage of her life. You're role is as important as ever, it's just time to guide her in new ways.
Do you have a lifelong passion that you haven't had time to pursue? This may be a great opportunity for you to take another look at it.
I wish you the best of luck. I know someday I will be facing the same empty nest. You must have done a great job raising your daughter and this would be a good time to congratulate yourself as well as her.
Life is an adventure, I share every day with my two beautiful children.