I've never bought texured brass plates, but used to make my own, etching the brass with ferric chloride, and using a paint pen as a resist and covering the backside with contact paper..information on this is available in Tim McCreight's book, The Complete Metalsmith, as well as many others.
I started with 20 ga brass instead of the much thinner stock I've seen available textured, and was able to use my plated with annealed silver or copper the same size as my etched plate almost indefinately, using a smaller recieving piece made marks in the plate and wrecked it.
I also used many of the items suggested above, especially when I was teaching about the mill, just make sure you never use metal screen or sandpaper not sanwiched between 2 pieces of metal completely, as you could injure your mill!
The only reasons I don't use these techniques now, is that I don't own a mill, and when I have access, don't tend to use it much, and with the etched plates, it was a look I used a lot for a pretty long time, but my work has evolved away from that quite a while ago, and I rely much more on simpler tools like destroyed hammers and very prcise us of my saw.
Hope that helps. Good Luck.