“save for a rainy day” = save something for the future when it is needed
“Instead of spending money on a new TV, I’m going to save it for a rainy day. You never know when you’ll need it.”

“a dime a dozen” = very common, usually inexpensive, and easy to find
“I bought my dad a new watch at a flea market, and I thought I got a great deal only to discover they were a dime a dozen at other places.”