Any surface that looks/feels flat will be good enough for this use case, no need to find a bit of glass. Most table tops will do. You might need something better for flattening heat sinks, but for 3d prints you don’t need to be that accurate. The plastic will deform far more under light pressure then the difference in any relatively flat surface you can find.

If you have one then there is no harm in using it - but also not need to explicitly look for something that flat. Any table will likely be good enough.