It was not uncommon a while back to see these being offered. Since then, other mag-fed MBRs have become available, and there is little need to go through the expense of converting an M1.
Make no mistake, this conversion, done right can be costly, given the amount of machining, new parts that must be properly heat-treated, and final re-finishing that is required.
No doubt there is someone out there who still does this.
Personally, I would advise against it, as you would be irreversibly altering a perfectly good M1 when there were plenty of other good and viable choices available.
Not to mention the fact that if the M1 alteration is not done just right, you've now ruined the M1 also.
The Italians used the BM-59 series of mag-fed M1s, but that was an arsenal conversion, with plenty of knowledge and technology behind the effort to see it done right. They are expensive (read uncommon), and the mags even more so. Sketchy parts availability and mag cost/uniqueness make this a collector's item, and remove it from the "go-to" category of weapons.
ETA: It should be noted that the BM-59 was only chambered in 7.62 NATO, and never in .30-'06.