Ammo is all over the place, online. No shortage, just a bit pricey for calibers like 308. But ammo is plentiful.
I prefer paying cash, over the counter. Apparently, so does everyone else here. Online sales are easily tracked, and if you obtained an arm via private sales (no records), can tip off TPTB to what you have by the calibers you buy.
Some things are best kept under the radar.