Tuba Peter (see related link) has a good variety or pops music and theres also many selections of classical pieces.
Nowhere.
Finding sheet music for solo Euphonium is rather difficult. If you read bass clef, it may be easier to simply find trombone sheet music and play off of that. If you read treble clef, go with trumpet sheet music. Both are Bb instruments like the Euphonium, and have similar tonal structure as well, so it wouldn't be too difficult to use those instead of Euphonium music. If that's not an option, websites such as sheetmusicplus.com, jwpepper.com, and musicnote.net can be a big help. Just search for common music publishers (Hal Leonard, for example) and see if they have online store fronts.
8notes.com
MakingMusicFun.net features a collection of free sheet music for the recorder in the Free Sheet Music section of the website. In the Free Video Lesson section of the website you can find step-by-step lessons to help you learn several of the songs. See link below
Ichigo's sheet music
Where can you find FREE sheet music for pure imagination?
Free sheet music for it's you Lord we worship by Paul Morton
See if your public library bought sheet music. That is free and legal.
You can get it at 8notes.com i get most of my sheet music there
There is a lot of websites where you can get free sheet music. Load.CD is one of them (all legal)
Since the music to Les Miserables is copyright, free copies of the sheet music are illegal. Without endorsement, you can purchase sheet music for this musical, below.
Piano Sheet Music Online offers free piano sheet music. SheetzBox also offers free online sheet music for pianos. One can also go to their local music store and they will have a variety of sheet music available for sale.