Welcome to the forum and congratulations on your new acquisition. I have two MilPros - a 40S&W and a 45ACP and lucked up and found extra mags at MidwayUSA a couple years ago but have seen then hanging on the walls at gun stores, shooting ranges and so forth. They might be tough to find with the purchase panic surrounding firearms over past 2 1/2 months or so but just keep checking. Several online websites other than Midway you might check are Brownells, Sportsman Outdoors, Cabela's etc. Some will have real time availability status updates so you will know of shortages at the time of order; others will notify you of that fact at checkout. You might also check with TaurusUSA too.
I am going to move this to the MilPro Subsection as you will likely get more helpful remarks there.
They show to have the 10 rd. but no 12 rd. = Price $38.99, which is same price for PT24/7Pro 17 rd. 9mm they do show, which will work in the PT111MilPro. Their PT140Pro mags are more reasonably priced at $29.99 and should be same for the PT111MP since basically same - guessing they feel they can gouge a bit.