In the keybindings you can bind keys directly for buttons on any action bar.

Alternatively you could use a bar AddOn that has "paging" so that when you are holding, for example, the shift key it switches to a different action bar, and then switches back when you release it. I believe both Bartender and Bongos have that functionality.

Another option is to bind keys directly to the spells themselves, skipping putting them on any action bar at all. Either BindPad or SpellBinder are AddOns that will do that.