Looking for information on how to pre-lease our first mixed use retail / apt.

I spent about 12 years in retail financial planning before I had not only "had enough" but also had enough confidence to do something about it.  

Check ICSC ( International Council of Shopping Centers ) website and directory for a tenant rep leasing broker.You have to find a broker that really likes commercial leasing because not much money in it compared to transacting.I don't touch property management or leasing broker.200,000 gross income center to manage at 4% fee is 8,000 a year if principal broker keeping it all.Leasing a Starbucks tenant to a center maybe make 15,000 to 20,000 commission.Sell a retail center for 3 million maybe make close to 100,000 commissionThese are amounts for principal broker for their own company an they keep it all pretty much.

My ultimate goal would be owning commercial properties (retail stores and warehouses). 

While the MLS is primarily retail listings at market value (or above), there are deals to be found. 

Here you go: "57.0 DISTRIBUTIVE TRADES AND SERVICES:Includes assets used in wholesale and retail trade, and personal and professional services."

We manage a commercial property (retail, multi-tenant) and had a fitness tenant (a franchisee) who went dark late 2019 so we terminated the lease (they still owe us 3 months of rent). 

Problem is that laminate and stuff like that is way more expensive in the retail market.

In general you can't get to this goal by buying a retail property off the MLS because if you could, everyone would buy them (i.e. demand would be high, supply would not increase and prices would go up)  Banks in general will finance around 80% of the fair market value of the property, so you need to pay 80% for a property worth 100%. 

I don’t think you’re going to get any cash flow out of that house and you can’t build a rental portfolio out of house as you pay retail for.