1) Acquisition is a lot more than money. This isn't Dark Heresy. An Acquisition check represents using status, trade agreements, reputation, favors, and even huge sums of money. When your characters send out their factors looking for rare goods, money only changes hands on the most common of goods. Often your factors are arranging shipments of goods, transport of goods and personnel, trading favors or interests in various endeavor, or even access to rare creatures and goods harvested from the Rogue Traders travels. Sometimes they are simply calling in favors or calling upon ancient oaths to obtain the assistance and goods you seek.
2) There are lots of worlds that produce STC goods up to lascannons with only minor oversight by the Ad Mech. To do so they have to have the STCs or manufacturing schematics of some sort. Therefore, those STCs aren't that rare. If Hive Worlds pump an item out then it isn't likely to be too hard for someone with more wealth, status, and power than a Hive World governor to arrange the right to produce such items (obviously including access to the STC). A good portion of the less exotic goods in the Imperium are made on Forge Worlds, usually by corporations outside the Ad Mech. Much of the time those factories have a rather small Ad Mech presence. For the more exotic goods you will have to arrange the contacts and convince the Ad Mech that you can be trusted with such a holy STC, as well as possessing hallowed factories and skills to produce such goods. Explorators have a much easier time obtaining STCs for exotic goods, as the Explorator fleets are notroiously well equiped and are directly involved with the Quest of Knowledge. Synthetic Muscle Grafts are so commonly used in the Ad Mech that I would find it likely that any major Mechanicum holding has them. The only difficulty is seeing if the local Tech Priests can be traded with, convinced, or coerced into releasing said STC to a holy priest seeking out Knowledge for the glory of the Omnisiaah. I generally assume that RT characters have access to any reasonable STC that is Rare or lower. At their level of personal power and experience, a Explorator would almost have to know many of them to do his job.