If you're looking for expired domains there are some great services out there. Best of all - you can start with most of them for free.
PBN HQ Domains is a combination of database and service. They scrape the Internet for the domains but they also do manual spam checking on those domains. PBN HQ integrates with Easy Blog Networks API and will automatically deploy the domain to your EBN account.
Domains are sold from $10 to $20 (without registration fee). You can browse their database for free.
Hammerhead Domains has a free marketplace where they show you daily scraped domains. You have to do your own filtering and research.
Their premium domains are sold from $50 to a few hundred dollars (registration fee included). You can buy topic packs for $97 for 50 domains (without registration fee).
Pure Quality Domains is a full-service broker. Just request the niche and number of domains and they take care of everything. Domains have minimum metrics of 15 DA/TF/CF and go through an 8-step manual quality review.
Domains start at $40 and go down to $15 for bulk orders (without registration fee).
Dominating PBNs focuses on premium auction domains with contextual links from authority websites. They guarantee a clean backlink profile.
Premium domains run between $200 and $3000 and are already registered.
We highly recommend you signup with all of the above services and see which has the best domains for your niche.