All of them have non-expiry free tiers (5 downloads a day each except Miatrix, which is two a day) except NZBGeek, which has a 3 day, 15 download trial. Of the recommended indexers with free tiers, DrunkenSlug and Tabula Rasa (as well as DogNZB and NinjaCentral) require invites to join. Each one has a premium tier that gives you more downloads for paying per month or per year, of which NZBGeek is among the cheaper options. Using usenet as a complement to what can be more easily found on public and private torrent trackers, I've found the free usenet indexer tiers to usually be enough. Another benefit of having accounts on multiple indexers is that as some releases may be indexed by one and not another, it can be helpful to search for releases on alternate indexers if your preferred one doesn't return any results.

Note that you'll also need also need to buy a usenet provider subscription or block plan to be able to download the indexed releases, as well as download a usenet binary newsreader such as SABnzbd or NZBGet. As having providers on at least two nodes mitigates the risk that a release is missing from either, I've found having a Frugal Usenet subscription plus a Newsdemon block plan to be a relatively low cost combination. Something to note when picking a usenet provider is retention, i.e. how many days worth of releases they have available. To download releases that are more than a few years old, you'll need a provider with that many days worth of retention.