Okay, let's take a serious look at the options we have for a source of rainbow shells...
- A fish pond, with 9 or 10 rainbow trout has a 2% chance of providing one... that's less than 2 per year on average, so not something you want to count on
- A gift at the Feast of the Winter Star, if on the small chance Vincent or Jas drew your name, and if from their options that's the one picked... and is only once per year, so you want to count on this one even less than the fish pond
- In the mail from Demetrius, and while the chances of a gift in the mail aren't super high you can increase the chances by increasing friendship
- On the Beach during the Summer, as already mentioned, and yes, this is easily the most reliable way to get a few with the only drawback being that it can only be done during the one season