Of course they are out there. Pretty much everything happens.

It isn't common enough for anyone to be like "hey I am going to meet a significantly younger person, become friends with them while they are in a relationship, and then they will fall in love with me during that".

Did you learn details about those relationships? What did they have to offer each other? I think there is usually more than just love being offered to each other (hence the stereotyped terms of sugar daddy, gold diggers, boy toys and Cougars). And did you find out how they met?

Your chances are significantly reduced if you aren't able to offer more than just love, and if you are expecting to meet one of these rare women in a random way.

It'd be much easier to be more realistic and find ways to meet women your own age. It's.... Kinda insulting to women if you are so shallow that you're only looking for women young enough to be your grandkids. And yes, it will creep almost all of the girls you meet out when they learn of your infatuation with them. If you absolutely can't get past this kind of fetish.... Then you need to find somewhere to meet girls looking for men older than their dad. Meeting them randomly isn't going to happen and you're just setting yourself for unneeded pain.