IMSLP stands for the International Music Score Library Project. It is, without exaggeration, one of the most useful and far-reaching resources on the internet for all practical musicians. The library is very well organised, with comprehensive indexes, so it makes finding printable copies of string quartet music really easy – and it’s FREE.
For a lot of the works for string quartet, there may be several versions in the library. Usually there is a scan of an early published version, but for some works there is also a version which has been typeset to modern standards by a volunteer.
A snag with downloadable versions of sheet music is that it might be quite tricky to actually print it out. Some of the versions may be in ‘music’ size (12 x 9 inches), which won’t fit on to an A4 sheet (11.5 x 8 inches). You will need a bigger printer, or accept rather cramped music. Always use good quality paper, at least 100 gsm, otherwise print-through can be a problem. And pay attention to page breaks – the edition might be laid out with convenient rests at the end of certain pages, to allow for turning over. If there are 4 bars rest at the end of page 1, make sure you leave the left page blank, print page 1 on the right, and then you can turn. Hopefully, there will be a handy rest (or end of movement) at the end of page 3 !