Preparation can be entirely free. The Ministry's formation civique site publishes the official lists of knowledge questions for the multi-year card and the resident card, with revision sheets by theme. It also warns that paid sites claiming to be official are not run by the Ministry.
Two free pages in English by Thibaud Lepan (Éditions Au Texte) are worth a look. The registration guide explains booking, exam day and the exemptions, quoting the official page for each point. The official questions answered in English take one question per page.
If you want a structured guide on top of that, the same author sells one in English and in French. Its situation questions are practice scenarios written by the author, based on the official themes: the Ministry does not publish the situation questions, so no guide can reproduce them. Get the English guide, or the French version. The code EXPATADMINHUB takes 10 % off both.
Disclosure: these two links are affiliate links. If you buy through them, ExpatAdminHub receives a commission, at no extra cost to you. The free resources above are enough to pass, and the guide is a convenience, not a requirement.