My guess is local banks and credit unions are the most likely source. They keep the loans on their portfolio instead of selling them like the mortgage companies.
Rate locks are backed by rate swaps. The cost of purchasing a rate swap ultimately comes out of your pocket in the form of additional rate on the loan (the lender can add overhead of course). The cost of rate swaps has doubled in the past 3 months and quadrupled in the past 18 months. I believe it's currently around 3% on 10 year loans, so a 60 day lock on a $500K 10Y mortgage would cost about $2500 while a 200 day lock would cost over $8000.
One interesting hypothesis is that the focus on fixed-rate 30Y mortgages is fundamentally destabilizing for the US and world economy, because the stability and optionality of 30Y mortgages is paid for by added volatility of the 10Y debt market through those same swaps. Per this theory, US 30Y fixed-rate mortgages are effectively subsidized by the rest of the world. https://byrnehobart.medium.com/the-30-year-mortgage-is-an-in...
I do wonder, if we just had 10Y mortgages, would home prices just crash? People won't buy what they can't afford (I mean, sure, they do to some extent, but not if their monthly payment would double), so sellers might just be forced to sell for less. The first "generation" of 10Y-only mortgages would be disastrous for current owners, as their existing 30Y mortgages would be underwater. Or maybe people just would stop moving...
I just glanced at the article you linked, and realized I'd read it back when it was posted, and agreed with its conclusions. I just don't know what the solution is.
Going back to the Canadian mortgages, they have terms of 5-10 years but they can be amortized over up to 25 years. So their standard practice is effectively a balloon payment of up to 80% that is expected to be refinanced at rates in effect at the end of the term. This still financializes housing over up to 25 years (similar to the US) but without subsidizing the interest rate optionality.
In the US, balloon payments are heavily discouraged by federal regulation on what kinds of mortgages can be financially supported by the government.
The downside (for the borrower, anyway), of course, is that if you do live in the same house long-term, you'll have more-frequent forced refinancing events that might result in an unfavorable interest rate. But that otherwise seems pretty reasonable, and arguably (as the article you linked points out), the ability to "lock" your interest rate for such a long time isn't great for the market.
I suspect home prices won't fall by as much as you think they will. I suspect institutional investors will become major buyers and lease out those units; which would crash homeownership - at least in the popular metros