Bongwater
Walk of Fame Participant
The big three produced their own custom cuts, most being instrumental arrangements of pop hits. A number of the smaller ones grouped together to do custom music, although not as many cuts and not as prestigious a set of orchestras. Almost all were done in Europe.
Europe makes sense as many of the lower budget American record labels (and a few major label subsidiaries like RCA Camden) used them from the '50s to the '80s. If I'm not mistaken, 101 Strings was not one orchestra, but several under the moniker and all were European. The European orchestras were not only very talented, but dirt cheap too because Europe was still recovering from World War II in the '50s and '60s and American money was like pure gold. They could play any arrangement you gave them for a flat rate (no royalties.) And there were literally hundreds of them. Though when the recordings came stateside, the name of the orchestra was usually changed to something fancier sounding. Or at least pronounceable before getting on vinyl and into the $1.98 record bins at the drug store.