I ended up buying a few thousand of the Euro sized black Mayday card sleeves that everyone recommends. After using them for a few months, I've concluded that they're... okay, but lacking in quality. The cut is not a consistent length or quality, leading to some cards being shorter than others, and the finish on the bottom cut line varies in quality (straight va burred). Many of them came with creases through the middle that made them useless, as the crease would indicate which card it was.
After a few months using these sleeves, the tops of them have also wrinkled a lot, which is disappointing. More used cards are distinguishable from less used cards - which was the whole point of buying the sleeves in the first place!
Does anyone know of other, higher quality Euro card sleeves? I'll pay more for them, no problem, I just hope there are some solid quality sleeves for Euro size cards like there are for MTG size cards. An opaque back of any color is also preferred.