There are two editions available through Cimarron Music, both with options for PDF download: (both are for euphonium & piano)
The original edition: https://www.cimarronmusic.com/fantasiadiconcerto
Version arranged/edited by Bryan Doughty (as I understand it, this is an update to the original to correct some inconsistencies): https://www.cimarronmusic.com/fantasiadiconcertodoughty
The Doughty version includes both treble clef and bass clef solo parts. The original version *may* have a solo part in treble clef? Though I'm not entirely sure based on the description. Dave Werden or anyone who owns this edition may be able to confirm this.
Willson 2900 TA-1 Euphonium - Denis Wick 4AM
Yamaha YSL-643 Trombone - Bob Reeves BrassArk 5G "Gladstone"
Yamaha YSL-8440 Trombone - Denis Wick 5BS
VMI 3301S BBb Tuba - Schilke Helleberg
Past:
York Preference 3067 Euphonium - Denis Wick 4AL
Benge 165F Trombone - Benge Marcellus
Wessex BR140 Baritone - Denis Wick 6BS
F.E. Olds Special Trombone (ca. 1941)