I recently learned how to use the BNF online and in Paris. For those who have not used it:
You can access the catalogue here:
and for example type in Mustel. If you follow the links you can order everything they have online, and you get a pdf. However, it is not cheap. You can also go there in person, get a library badge for one day, and copy the music or even take photographs with your mobile phone (the composer has to be dead for some time, I could not order Joseph Bizet, but most of the harmonium music is old enough....).
Now the good news: it looks like they upload everything they digitalize for a customer on gallica, so that all of us can use it....
I recently ordered Loret:
And H.P. Toby:
I also ordered some other pieces by Toby, so maybe it will also show up on Gallica soon.... I hope you have fun with the music...