i use a granite countertop scrap with sandpaper
that is where i got mine, the shop closed after the economy tanked in 08. checked a small (14"square) piece at work and the surface is dead flat. you can usually find granite tile at big box store for less than $5 for 12"square, but it will be thin so you will need to mount it to something so it wont break. a couple pieces of granite is nice to have around, only surface I know where you can set red hot blade and not have to worry about damaging surface, an advantage over glass. I also use mine with abrasive sheets to check bevel flatness/eveness when doing final blade shaping and sharpening ; and with clover abrasive compound to lap.you can also find a counter top installer. They have what they call remnant. I got 4 pieces of 3'x2'x2" granite for free.
a platen on a grinder is the last thing you want to use if you want something flat. The belt puckers at both the top and the bottom rounding (slightly I know) over both top and bottom. A disk is several times better than a platen. After that, good technique on a surface plate will take it home.