One way to get the Glass Frog is to breed different frogs in hope of getting one. There is a very small chance that the baby will be one. You can also look for them in the Pond, and breed with them,...
Though most people say they can only be as rare as the level you are, they are wrong. i am level one with a frog that has the rarity of 10. You can wait until Christmas when they give out free stuff,...