Can anybody provide a reliable source that gives data on retail markups? My end goal is to make a fairly high-level statement along the lines of "a typical retail markup (across all industries) is X%", or "the average markup in industry Y is X%", so fairly aggregated data would be fine.
I recommend the Comp-Net database. It is a firm-level data for European countries. It includes a large range of variables, including mark-ups:
Check the documentation for more details.
For the US, take a look at this paper. The authors use three different sources of markups: Compustat, NBER Census of Manufacturing, and IRS (Inland Revenue Service). The first two are micro-data whereas the latter is aggregate at industry level.