Try large hotel chains Jake.
Some have what you describe, but worded "Sanitised for your protection", so you feel safe using a WC that thousands have used before you.
They seem to be polythene, or paper, and sized to slip over the seat and cover together, but I'm sure they're not adhesive. Perhaps someone from these hotels could let you buy some, or tell you who their supplier is.
Failing that, how about a plastics printer? You know, the folk who do carrier bags. Give them the size you need and what text is required.
Last resort - ask the builder where they've seen, or used, them before. No, ask them first. They've obviously had this on other sites.