In order for a purveyor/ slaughterhouse to control his sources of beef he must be the purchaser or grower of the beef. That is to say that the purveyor / slaughterhouse must have suffecient outlets to market his kosher and his non-kosher as well, as almost 90 % or more of the beef must be sold off as non-kosher (when you have a 20% kosher ratio-Glatt would be even less of a ratio).
Every Purveyor / Slaughterhouse must therefore operate on שבת ויום טוב וחול המועד including the trucking, plant (maintanence) etc. which involves many Halachic issues which we don't want to get into.
So actually we are left with a problomatic scenario; A Glatt / kosher purveyor that wants to control fully his beef sources must become a first & foremost a Non-kosher purveyor of beef, and in order to compete with the other larger non-kosher purveyors of beef he will also sell directly product labeled as Glatt or kosher (at a somewhat higher price) in order to make up the difference of his loss on the non-kosher, but he is actually owning & operating a NON-KOSHER slaughter-house,
(selling Glatt & kosher beef & poultry as a "side business").
The big question is "What does the strictly Glatt kosher purveyor do"? Most of the Glatt purveyors contract with the Non-kosher slaughterhouse to only take the small percentage of Glatt beef, the other 90% + of non-Glatt beef or non-kosher beef is left for the non-kosher slaughterhouse to get rid of. Therefore the Glatt purveyor is at a disadvantage as he can't control completly his beef sources. On the other hand he doesn't have to concern himself with Halachic issues of Shabbos, Yom-Tov, Chol Hamoed, מיסחר בדברים אסורים etc.
Allways check what type of Glatt purveyor is your supplier.