There is no 8mm Lebel revolver brass available in North America AFAIK... it's all trimmed down 32-20 brass. I ordered mine (untrimmed) from Track of the Wolf in the States, but it was on backorder for a few months before they finally shipped it out. If you want real 8mm Lebel revolver brass, there's a company in France (or Belgium?) that sells it... for big $. It all comes down to the headstamp, really. In other words not worth it, unless you're a diehard collector.
The best way to trim 32-20 cases would be on a lathe, but a case trimmer will do also. Personally, I'll try to anneal after trimming next time (before final resizing with the die), because the 32-20 brass tends to form an elevated ring in the mid section of the case when sized as is. I guess this is due to a lack of flexibility of an un-annealed trimmed case (the kind Tallman was selling). I tried fireforming a bunch afterwards to get rid of the ring, but I guess either my load was too weak or the ring is too strong.
Either way... annealing is recommended to prolong case life.