A good question.
Most rentals will cost you as much, or more, or at best, just a bit less then buying a birth pool.
One way to recover some of the money you spent on your birth pool is to offer it for sale to your midwife for 50% of what you paid for it. It is a win-win for both of you.
Another way to save money is to talk to your midwife to find some other clients who have births at least one month or more after yours. The two, three or four of you can share one pool and simply buy additional liners and accessories.
OR… use your birth pool as a personal spa. Relax as needed in the comfort of your own home in warm water when you have back discomforts or when you want to spa with a good movie in the living room. Who says water must stay in the bathroom or outside?
Use your birth pool as a play center for your toddlers.
Donate it to a family who can not afford to buy or rent a birth pool.
Blessings in birth,