McD’s sells ice for $1, and you can get it through the drive thru.
If you want long term use, blocks last longer. For a day or two use, cubes are fine.
I have read that arctic-ice blocks work amazing, I have them on my list to try next. There are different models for different uses, so make sure you pick the right type for your intended use, or they don’t perform as well. For instance, one works at freezing temps, best used to keep foods that are already frozen, frozen (ie ice cream and frozen pizzas on ride back from grocery store). However, that block will not perform well at objects that are above freezing.