Congratulations! I think that I saw this rifle on their website - i was surprised that they had DRs too!
Kynoch has a 3" and 3 1/4" Nitro for black load and they may be a starting point to contact for advice. These loads suggest that 440grain bullets are made by Woodleigh and you could buy brass directly from Kynoch at least.
You can always get alot of advice from Wright's book "Shooting the British Double Rifle" too
Hope that helps