Ask in your Buy Nothing group. I see them posted often there, but someone may have one in their garage.

Phoenix bikes in Arlington. A very cool program

You get what you pay for. The expensive bikes shift smoothly like butter, they are much lighter in weight, the stores will ooze over you for a perfect fit and give you personalized attention and customized seat heights, adjustments, knee analysis, etc. The big box stores will sell you the heavier, clunkier bikes with no customer service.

REI

Used bike store. We go to the one at the corner of Washington Blvd and Pershing in Arlington.

Our teens have Schwinn from Target and Nishiki from Dick's. They've been good quality and lasted well. However, bike costs did really escalate in 2020 and haven't come down. If you can get a high quality brand new bike from a local bike shop for $600, that's not bad. The upside is when you buy local they can help with fixes when things come up. Our Target/Dick's bikes were like $400-450 so not such a substantial savings from $600.

Walmart, Target

Guardian

Felt bike

Up & coming brand, wide variety of prices.

We got our 12 yo that for <<$800

Online Trek store get their Flex model line or something

Free delivery

Wheel & sprocket trek stores

I bought one for my kid off Facebook marketplace.