These are my go-to places to get COVID PCR tests in Washington, DC.
DC Health: five or six sites every day but Sunday, check that link (and scroll down a bit to see the handy daily list) to see what sites are open today. Results usually come back in 48-72h.
Sameday Health: a few different testing options, including two billed-to-insurance options ($0 out-of-pocket for <40h results, or a rush fee for <16h) and two self-pay options (one for <16h turnaround and another for <1h turnaround). Only has locations in Georgetown, Old Town, and Bethesda, but generally has great availability.
Curative: a bunch of locations around DC, and generally has OK availability with a turnaround time in 24-36h. But we've had a few issues with them (a site that decided to open an hour late and just push all appointments without notification, another site that fully-booked on a day they weren't actually going to be open), so we're a little sour on them right now.
Grubbs Pharmacy: beloved on Capitol Hill, great availability, and <24h turnaround, but they don't bill to your insurance; it's up to you to submit for reimbursement.
CVS and Walgreens: a lot of both pharmacies' sites offer testing, but I've found availability to be pretty spotty, and the process of booking an appointment with either is laden with a ton of up-front questions before you can even see if they have availability.