Boy are you in luck! A quite decent indexer that is normally invite only just opened registrations for what I assume is a very limited time: DrunkenSlug. They have a free tier which is limited to 25 API hits and 5 downloads per day. For €15/year, that gets bumped up to 1000 API hits and 100 downloads per day.
Another popular one that doesn’t require an invite: NZBgeek. They don’t have a free tier. I don’t think they have any hard limits on API hits and downloads. Pricing is $6/6mo, $12/yr, $40/5yr and $80/lifetime.