Our MLS shows how/if a home was financed when it's closed.... but it's a laborious process to go through property by property.

HUD and census but they are historic as of last year 

https://www.census.gov/construction/nrs/pdf/quarterly_sales.pdf 

Lenders do monthly reports of loans closed but wont show cash https://www.ffiec.gov/hmda/pdf/2021Guide.pdf

Corelogic can give you numbers email me the county and time frame let me see if they will provide 

NAR surveys but it's not accurate

ATTOM Data has all the 1-4 that were appraised with a lender and some with cash. However, some cash buyers don't get an appraisal

 https://www.statista.com/statistics/185213/us-house-sales-for-cash-since-2000/    County records are public in most states. Your title rep can always get the numbers

The only way I know is to run an MLS report for cash purchases.

I thought I saw at the bottom of closed listings on the MLS mention about how the home was purchased (financed) and if any concessions were made. As far as a collective source, I know not

Yes, our Local MLS does show a " How Sold" category which can be added as a search criteria. Cash, Conventional, FHA, VA or USDA will show.

Our MLS can easily tell us that. One of my market reports I used to do was % of cash, FHA, Conv & VA.

Here (AZ) a property profile would show lender and amount encumbered, but for a large area, hmmm . . . you may want to check with title company and see if they have stats!
Our MLS closings will show 'how purchased' and I just added it to my criteria fields so if I pulled up 'cash', 3049 listings total past 180 days became 891 bought with cash.
So a LOT of financed deals - 3 to 1, roughly! 

Your question made me dig into our MLS and discover that it is possible to generate a report for financed vs cash sales in various communities. For instance...since April in 1st in Fremont, Ca. our mls reports 191 sales of which 13 were cash purchases. 

with a lot of research in my area the public records show if a "security deed" is recorded or not.  But easy enough to find the data for those in my MLS

That criteria can be searched in the regional MLS.
The math however is not DFU.
total sales.
Cash sales
Financed sales
The numbers never null so a bit of swag is needed.
In my slice of paradise, from my calculations 3:2 in favor of cash.
But it gets cloudier. Ribbon and Knock are two services that help to muddy the water a LOT by turning financed buyer into cash, TEMPORAIRLY. The good news is total sales are down and dropping meaning one CAN spend the time digging and digging and digging and use those findings for at least 6 months' worth of marketing. No other agent will invest the energy.

I know that our mls keeps track of this.

We can search for cash financing and see how many paid cash versus financing.

In my MLS, you can pull data for sold properties which includes how the property was purchased.

For example, so far this year in Kalispell (through the end of March), 27% of house sales were cash, 61% were conventional, 5% FHA and 6% VA.

The same time frame, for land, has 63% cash, 31% conventional and a few owner financing or other.

If it's outside of MLS I don't know how to get that information.

Sounds like the more experienced agents have provided some great suggestions.

Yes, it would take a lot of research and resources though.