Most of the time I do all my printing myself. I use a Ricoh SP 6330n oversize B&W laser printer, but I’ve owned HPs before in the past. My standard parts paper is 9.5" x 12.5" which is the size many older manuscript brands used such as Associated Music in NYC. 9.5x12.5 is a particularly convenient size because 25x38 is a common size for paper suppliers, and they can cut one sheet of that into 8 sheets of 9.5x12.5 without any waste. 9.5x12.5 is also a MOLA approved size, while 8.5x11 is not.
There are about 20 of us copyists/arrangers/composers in the NYC area who all go in on a bulk order a couple of times a year in order to get a better rate from the supplier. I’m not the one placing the order so I don’t remember all the details exactly, but I seem to recall the supplier gets the 25x38 paper in 500 sheet cases, so our order of 9.5x12.5 is in multiples of 4000 sheets, which the supplier boxes up 2000 sheets to a box. I think we’re using 100lb Text weight (remember paper weights are not equivalent) off-white paper for accordion-style parts. The brand has changed a couple of times so I don’t recall which one we are currently using.
Some people prefer to print 2-up on 19x12.5 paper as it is less binding. I just use 9.5x12.5 (we don’t order any 19x12.5) and then use acid-free artists tape for accordion-style parts. For 11x17 scores and ring bound parts I usually use Hammermill “Color Copy” 32lb paper and just cut it to size myself if needed with a paper cutter.
When I’ve had to send scores out for saddle-stitching, I’ve used Subito before: https://www.subitomusic.com/services/printing-production/
I’ve heard good things about Black Ribbon but have never used them personally: https://blackribbonprinting.com/