There are a load of trombone and band pieces from the late 1800s and early 1900s. Most are fairly easy to play and are much shorter than the Rimsky or the Guilmant (and that section in 6 flats is poison to a lot of amateur bands).
Look for these Arthur Pryor solos:
Blue Bells of Scotland
Thoughts of Love
The Little Chief
Or Clay Smith:
The Water Witch
Gardell Simons: Atlantic Zephyrs
Henry Fillmore: any of the Trombone family. Lassus Trombone, Shoutin' Liza Trombone, Bull Trombone, Mose Trombone, Sally Trombone.
Fred Harlow (probably a pen name for Henry Fillmore): Air Varie The Old Home Down on the Farm, The Wanderer
DeLuca: Beautiful Colorado (often presented as a Baritone Horn solo)
Most of the Herbert L. Clarke solos for cornet can be played on trombone:
Stars in a Velvety Sky
From the Shores of the Mighty Pacific
Bride of the Waves
You can find full parts for some of these on www.BandMusicPDF.org
since most of them are Public Domain.