A lot of the windows and doors in PR come from Home Depot. Sizes should be standard, measure the current windows and take a photo then go to HD.
Your measurements may not be exact given that the window may float within the hole or part of it may be inside the cement, I would not worry it is not exact to a 1/8 of an inch. You can always return them.
also if you are getting a regular contractor or a HD contractor to install them, they will measure and tell you what to get