I agree with BSG. I do not believe there is a substitute for the thick leather in a Brooks. It's what gives the saddle stiffness, yet the pliability to mold to the users contours overtime.
However, if the width and the springs are what appeal to you, then there might be some other options out there for you. Most of them will be cruiser-type saddles, though. These, for me, tend to be overly padded and uncomfortable for more than casual rides on bikes that let the rider sit fully upright.
FWIW DH's friend, who is staunchly vegan and a LBS owner, rides a Brooks. If there was a substitute, he would know about it.
Everything in moderation, including moderation.
2007 Rodriguez Adventure/B72
2009 Masi Soulville Mixte/B18
1997 Trek 820 Step-thru Xtracycle/B17