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I've never been to Costa Rica but speaking from experience as a lifelong Floridian (which has the same c**kroach problem).
If you read the reviews for any Airbnb in a place that has native c**kroaches, you'll find all travelers from outside of tropical areas tend to complain about them. In many tropical places, there is no such thing as c**kroach free, as they can and will get in, especially if it rains.
There is, however, a difference between a visitor seeing a c**kroach and a c**kroach infestation.
If it ends up raining while you're visiting, you'll probably see a couple of c**kroaches in the place you're renting, especially if it's a house vs. a hotel. If you're not on a ground floor and/or away from outside doors and windows, you most likely won't see many, if any, c**kroaches even when it rains.
An infestation is entirely different, and that's what many Airbnb reviewers get confused about. An infestation is when many c**kroaches live in the house and reproduce inside. Most creepy crawly visitors don't stay for long, and if they do, they tend to die inside. However, if a house gets infested, that's a different story.
It shouldn't be too hard to avoid infested houses -- if it looks like it's clean inside, you should be good. You will though still have to be ready to deal with the lone c**kroach transiting through your property.
As for where to buy bug spray -- I've never visited Costa Rica as mentioned, but Costa Rica does have several Super Wal-Mart locations, and their Central American website does show that they sell roach spray.