Some countries say PCR negative Covid test required, but won’t specify what exactly. Does this mean you could go to a place like Walgreens or CVS for it?
The tests are good for travel, as a friend that traveled to Guatemala did use a pharmacy test to travel. Just specify that the results are for travel. But, good luck, like the one I got early in the month.
Two weeks ago I did the test at Walgreens, the traveling advice that other Hispanic friends gave me was that one needed a negative test made within 72 hours to travel. Before the test, the pharmacy reported on its internet site that the results would be available within 3 days, so we did the test on a Tuesday to get results within the 72 hours to travel on Friday.
The pharmacist told me after taking the drive-tru test that the actual time estimate was 3 to 5 days, due to demand…
Turns out that the early advice was wrong to travel to El Salvador in particular (almost all other Hispanic countries request it); the test was not requested by El Salvador since October, just the evidence of vaccination (and even that was not requested when arriving) So I ended getting the e-mail with the negative results… 2 days after arriving.
Coming back, the US does need a negative test, and there we also ran into a demand issue in one of the most reputed labs in El Salvador. After their ads reported results in an hour, they told us that because of demand, the actual test results would come in 24 hours…
I was resigned to pay more for a later flight when I decided to check for other labs, turns out that there was another lab in the same building that also made tests for travelers in one hour.
Negative results, using masks even there is effective too 
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So check for demand delays that could be there in the pharmacy just before the test, you should know about a lab location, with test results that you can use for travel as a plan B if the pharmacies come with demand issues.