Recycled steel if you can find any.

[Toolmaker51]

Home made lapping plate
The Home Engineer

I'll add a tip for those building a lapping plate of their own. Dry paper and wet or dry both work well.
In hand lapping, strokes taken in a straight line often results in the corners out of plane, even circular movements aren't ideal.
Proper lapping is best done in figure 8 patterns, and periodically alter the hold 90° too.
A good way to interpret attained degree of 'flatness' is easy. Take one easy single linear stroke; then re-orient 45° with another single linear stroke about HALF the distance of prior stroke. You will see fine, evenly distributed scratches 45° to each other; just like a properly honed engine cylinder. It sometimes helps to dye a few places with red or blue permanent marker [felt tip pen].