Are there any maps of taps where I can fill my water bottles (bidons) on long rides. I live in Scotland, but would be glad to learn of the existence of such resources worldwide.
Openstreetmap with the cycle layer certainly shows some. Here's one I was very grateful for in the French Alps. OSM is not complete but it's open source so you can improve it (I haven't added any taps, but I have added bike parking). This is a good approach, as it also shows cafes and shops where you should be able to buy water if there's no tap (flipping between the "standard" and "cycling" layers can show more facilities).
In the UK, the cafes on Cafe Network will almost all be very willing to fill bottles for customers - I normally plan on a cafe stop every few hours and take the opportunity then. Pubs and coffee shops generally will too. Some chain fast food places won't - they say they can't. This is problematic late at night when nowhere else is open.
A possible fallback in the UK is churchyards - there's generally a tap outside, but no guarantee the pipes aren't lead.
It's possible to query OpenStreetMap for drinking water by modifying the following link to give an appropriate bounding box (the last 4 comma-separated numbers):
value=water_point returns some more (e.g. next to canals, for filling boat tanks).
Of course some are on private land. I've seen campsites and schools showing all their water points. But that's fairly obvious from zooming in. The bigger downside is that many are missing.
In the USA free tap water is virtually universal, and I was able to fill up my water bottles at any fast food restaurant and convenience store I stopped at, all over the country. A switch on most restaurant soda fountains will dispense plain tap water. I also filled up at local parks and roadside rest areas.
In the UK, we used to pay tax for public services, now everything has been privatised but we still pay loads of tax. There was a time when most railway stations, bus stations, high streets all had drinking fountains. All of these have been removed, for people to buy plastic water and then throw in landfill. I had thought at one point the access to free clean drinking water was a defining mark of a civilised nation.
On a less pessimistic note, using a map to find water courses, coupled with a handheld water filter, or bottle filter should see you right somewhere like Scotland. They are surprisingly affordable, and some claim to remove 99.99% of bad stuff, seen inline filters for drinking bladders.