They were by subscription from MM directly. 2 titles a month and at 45 rpm. This got you numbered titles. Your number was the same across the series. However, the titles were also available to purchase from online retailers like Acoustic Sounds, Elusive Disc, Music Direct, and the now defunct SoundStage Direct. They were also available in some record stores. We're still battling COVID-19, but Jazz Record Center in NYC still has MM titles for decent prices, often more affordable than MM directly. Although that might have changed since people got hip to them having these titles for decent prices. For all I know, they could be all gone.
Anyway, the beauty of the online retailers back from 2007 to 2012 was that the coupons they offered had larger discounts and there were no "excluded titles". Today, it's 15% at most for a limited time and the best titles are excluded. The standard is now 10% off. When they still operated as a reputable company, SoundStage Direct regularly put out 25% off coupons, which made Acoustic Sounds, Elusive Disc and Music Direct follow suit with 25% off coupons. This was applicable to MM titles, so you could buy the 45 rpms for a little over $37. And even without coupons, the 45 rpm titles were $50 a title for years.
The 33rpm MM titles started at $35 a pop, then bumped up to $40.
The surge in demand has no effect on me. I got the titles as they came out, so I've never spent more than retail and in many cases, I bought them with coupons and discounts. The only titles I've ever spent more than $50 on are the SRX titles because the retail price is more than $50.