How to get the cheapest possible flight from Omaha, NE to San Juan, PR?

I will be flying to San Juan from Omaha in May. I need a flight sometime from the 6th-10th of May and I will return to Omaha during the last week ofJuly. I am on a very tight budget so I am trying to find the cheapest flights possible for this. Am I best to book a one way ticket each way? Are there any sights that you know of that can set me up with cheap flights? Any coupon sights? I am really desperate to find as affordable a flight as possible. Thanks in advance for any info you can provide me!