The various component suppliers like Midway, Graf and Son, Cabelas have 348 by W-W, Jamison , and Hornady. Many:out-of-stock.
Brass and bullets are sometimes found in the items for sale in outdoor or firearms sites. Also gun shows and
small newspapers may have 348 brass- along with gun shops.
I have a 5-die set for the 348 WCF to 450 Alaskan. Necking down and re-sizing 50-110 brass could be done. It does involve
a lot of cold working of the brass in the neck area. One might want to anneal the cases. Dies are not cheap and neither is 50-110 brass.
Recently- I found almost 70 new W-W 348 WCF cases reasonably priced-online-and I was'nt even really looking. Something to consider.