Go to Pyramydair, Airgundepot, Airgunsofarizona, to name a few. Those are the biggest retailers.
You can buy an Umarex Gauntlet for instance, for right around 400 bucks in a few different calibers. Then you'll either need to pair it with a hand pump, or a SCUBA tank, or a compressor. I use a little Chinese-made compressor branded GX.
Nicer options of course cost more money. But just like in centerfires we talk about here all the time, there are the secondary costs that come along with the gun purchase. And those have to be figured in too.
Most people consider FX airguns to be in the top three brands. But with those, you'll start at around a grand. Same thing with most of the big bore airguns