Usually, every few days, at least once a week, Apple stores get stock - stock that probably disappears pretty quickly. Can't guarantee they are getting Minis, but they probably are getting some. So you have to check several times a day, every day, to make sure you don't miss it.
Just go online and under buy, configure the iPad mini 6 order to what you want, then check the local pickup availability. Stores get a few in of different colors/configurations. Looking at this search when they get truck deliveries at different times of the day will show if what you want is temporary available. The more stores local to you the better the odds. If you see what you want buy it immediately. Also the trade in for a mini 5 is good as a option which will be a credit of your purchase. You have to do this multiple times for a few days to get what you want. But I did just this to get mine rather then using 4-5 week delivery.
On a previous iPad buy, the web site showed my local store as out of stock but the app showed it in stock and I was able to purchase it there that day.