While having a complete accoutrements set from the same maker is a nice personal collecting goal to shoot for, keep in mind that any particular soldier may or may not have his accoutrements issued that way...meaning from the same maker, except by chance. It is also true that they would not necessarily have been issued at the same time. Accoutrements were procured through Ordnance, not the QM Dept and as such they were not replaced on any sort of schedule (rather replacement was based solely on serviceability/wear and tear). This is true even if Ordnance issued the soldier a new "modern" rifled musket in .58 to replace an obsolete foreign or domestic smoothbore musket that was taken out of service. The bayonet would be replaced, but not the soldiers old accoutrements such as the .69 cal cartridge box, belt and scabbard.
E.A. Crossman was listed in Newark, New Jersey as a harness maker from 1861-1865, and listed at 164 Walnut Street (1861-62), 13 Franklin Street (1863-64), and 104 Railroad Ave (1865). The firm received US Ordnance Department contracts in 1864 and 1865 for 15,000 sets of infantry accouterments, 5,000 saber belts & belt plates, 6,100 carbine slings, 2,500 carbine cartridge boxes, as well as additional orders for halter and stirrup straps. The firm continued to supply the US Govt through the 1870s and employed about 15 people. Hence, that number does not make them sound especially rare however, the pieces needed for a "full set" may take some patient searching on your part. Probably two million accoutrement sets were made during the Civil War, so statistically speaking less than 1% were supplied by Crossman.