Listed on Hodgdon's site as .257 Weatherby Magnum.
Midway shows No$ler, of all places, as being the least expensive at $67.99 per 50. Followed by Norma at $34.49 per 20($154.99 per 100). Hornady is on out of stock, no back order.
"Need load data for this caliber. Also need Brass."
Your loading manual doesn't have data for the Wby????
I think I'm using 7828SSC in mine. Also have some old production AA870(0). Would not expect top performance from any powder faster than 4831.
Re: cases
If you can find the Norma made Wby ammo (around $30-35 a box), you get good cases and it's already loaded for the same cost as Wby/Nosler brass.