I have a soprano sax and have noticed that it is REALLY hard to find music for it. Does anyone know the best substitute to use?

You don't say if you play solo on your own or in a group.

Music for the tenor, being in B-flat the same as the soprano, will work with no problem. Also clarinet music can work although you might find that goes too low for the sax necessitating playing bits up an octave.
Alternatively read from vocal lines or other 'concert pitch' music & transpose it all up a tone - a very useful technique to learn.
If you play totally solo most music written in treble clef will work depending on the range of notes it uses.

Many trumpet parts also work well on soprano sax as long as it doesn't go too low.

if your ear is good check out Marion Meadows, Paul Taylor, Grover Washingtion, Kenny G to manipulate. The Chart "Peanut Dog" has a soprano part in it.

clarinet would be a good subtitute