You're not going to find any reliable sources of used brass. I know the new stuff is higher than average brass, but it's also higher than average quality brass. Norma brass is much better quality than Win, Rem, or Fed brass. It's worth it. As long as you do your part as the handloader, the brass will last many firings. When I bought my 257 wby, I just sucked it up and bought 100 pcs of new factory brass with it when I bought the dies. Sure it stings a little, but not counting the odd lost piece of brass in the field, I'll be passing that brass along to my son with the rifle and dies when I'm too old to hunt anymore.
You and your buddy should just split an order of brass and be done with it. (I know, it's easy for me to say, but for the record, I'm a blue collar wage slave with a wife and 2 kids to support, so I'm not rolling in money).
You could use 7 Rem Mag brass sized in your 257 Wby die, but I've never done it, and I'm NOT recommending it.