Depending on your budget, you may be able to find a Yamaha 301M or 202M (the marching baritone and euphonium that Bluecoats use, both large shank) but those usually go north of $1000 used, unless they are beaten up and/or need repairs.
The Marching Exchange on Facebook and Ebay has been listing some factory refurbished King marching brass for sale recently. They had a few lacquer King K30/SB30 hybrid marching euphoniums for $700 I think, those could be a good option for a solid marching horn at a fair price. They're large shank, and are "hybrid" in the sense that they have features of a marching euphonium, but are a little more compact and not as huge as the 1129 marching euphonium, or other euphs by Jupiter and Yamaha.
Best of luck on your audition! I hope you have a fantastic experience and that the season works out in some shape this year.
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)