What's the most "financially responsible" way to purchase physical copies of the Nana manga volumes (1-21)? (Patreon)

Since we're in the latter half of the anime, I want to start planning for the transition to the manga. Barnes and Noble only sells the physical manga individually for like $12 each. Any recommendations for more economical options? I was hoping it would be simple to find a boxset, but no such luck. Appreciate the help!