The Tour is one of the last great sporting events you can watch from a few feet away without a ticket. Turn up at the roadside on a mountain stage and that is the whole transaction.
Almost everything written about attending the Tour is trying to sell a package: a hotel, a bike tour, a hospitality seat on a climb. None of that is required. The route is published months ahead by the organiser, the roads are public, and the rules that decide the racing are published free by the governing body. Read those two and you can plan the trip yourself.
The reading list
Link-out only. Agencies and primary rules, not people selling you the gear.
The organiser's own site: route, stage profiles, timings and start towns, usually published the previous autumn. Everything you need to choose where to stand is here months in advance.
The sport's governing body. The rulebook covers time cuts, sprint classifications, team sizes and the equipment regulations that shape how the racing looks.
An example of a national weather service. Mountain stages are decided by heat and storms as much as by form, and a day on an exposed climb is a genuine exposure risk.