Where you are matters. And you do have to tell us more about the rifle.
"...375 Flanged Magnum..." Hi. Which one? There's a .375 H&H Flanged and a .375 NE Flanged. The H&H is a bottle necked case, the NE is straight walled. Bertram makes both types of brass. Neither is cheap. Shooter's Choice, in Waterloo, Ontario, wants $117.95 per 20.
Kynoch loads ammo too. There's a picture of both cartridges on their site.
http://www.new-kynoch.apt-sites.com/cartridge range.htm
Tradex wants $72 per 20 for Jamison H&H Flanged Magnum brass.
RCBS dies are a 'special order' thing. They run $142.99US with a 90 plus day wait at Midway, Stateside. They'd be more money and likely a longer wait in Canada.
Midway also lists Kynoch ammo. Sit. $57.95US per 5. No exports from Midway, but it'll give you an idea of which organ you'll need to sell. snicker.
"...should be able to order you some..." Subject to their supplier having any in stock. Not likely, but it doesn't hurt to ask.