Last night at Midnight Pacific time Apple started selling the new iPad mini with Retina Display on the Apple Store. As of this writing, the WiFi iPad mini is shipping in 1-3 business days, while the LTE models are shipping in 5-10 business days. There's also a personal pickup option to get your iPad mini with Retina Display in an actual Apple Store, although none that I've been able to find are showing any availability. Perhaps that will change throughout the day.
Apple's PR on the release also mentions availability through cellular carriers and authorized resellers, but until businesses start opening up we can't confirm any kind of availability. As it is, it seems like you're better off ordering from Apple direct, and if you can find one locally just cancel your orderâ€¦ That's what I'm doing, anyway.