Look into the possibility of a home equity loan to pay for the pool. If you have enough equity in your home, this type of loan offers flexible financing options and you can often deduct the interest on your taxes. Talk to your banker to explore this opportunity.
Talk to the contractor installing your pool to find out about financing options they have available. Many contractors offer their own financing for the home improvement projects they sell. The rates may be slightly higher than a bank, but it's still a viable option.
Check the rate and available balance on your credit card. If you have enough credit available, charge it to your card even if it's just temporary. You can accumulate points if you have a special rebate program and then pay it off with other permanent financing down the road.
Apply for a home improvement loan through a bank or credit union. Many lenders will make these types of loans for in-ground pools. The rate is slightly lower than a personal unsecured loan so you could save some money on interest if an equity loan isn't an option.
Ask a friend or relative to lend you the money if you have no other options through a lender. Offer to pay them interest and make sure you clearly agree on what the repayment terms will be.