If you are in US, here are some labs that I've used in the past for RT-PCR testing:

• Quest Diagnostics: can get sample at Walmart - > turn around time is about 5-7 days
• Pixel Labcorp: at home mail in test > turn around time is about 3 days
• CVS: in person collection at stores > turn around time is 1-2 days

The turn around time varies greatly depending on the # of active cases, and # of tests being run on a daily basis. But these are only my experience.

Also note that there are different variations of the RT-PCR test. The term RT is itself confusing as it can mean "Real Time" or "Reverse Transcriptase". After reading the indian govt guideline, I assume it to mean "Reverse Transcriptase". The coronavirus is an RNA based virus, so you need to use reverse transcriptase to convert to DNA before starting the amplification. There are a variety of different methods to amplify sections of RNA/DNA for testing purposes, all of which fall under the umbrella term Nucleic Acid Amplification Test (NAAT). These include Loop Mediated Amplification (LAMP), Nicking endonuclease amplification reaction (NEAR) and Transcription mediated amplification (TMA). RT-PCR is one of the varieties of this. However some labs might do a LAMP instead of RT-PCR. For all practical purposes the results from a LAMP, NEAR or TMA will be identical to RT-PCR. Most of the test results list themselves as NAAT, and don't mention RT-PCR anywhere on it. So be careful, the babus at the Indian airports generally aren't saavy enough to know the difference between all of these.

The final thing to take note of is that some labs use what is called Pooled Sampling. This means that they take the results of about 20 samples and pool them together. Then they run the PCR on that single pooled sample. If negative, then the entire batch is deemed negative. If positive, then they'll run individual PCRs on the 20 separate samples. This increases the efficiency of the PCR runs. I noted that the CVS myHealth report mentions pooled RT-PCR testing, but the others don't. Not sure how the babus would react if they saw that on a report.