Help Needed for Sourcing an Inclusion List in Programmatic Ads

Hey everyone, I'm in a bit of a bind and could really use some advice. We're currently using Prebid with IAS for content targeting and avoidance, and also wrapping our creatives for postbid verification. However, our client is insisting on an inclusion list. Previously, when we worked under a holding agency, we used their inclusion list, which was proprietary. Now, I need to find a solution independently. Where can I buy a reliable inclusion list of 10k-20k sites and apps? Also, for those of you who have experience in a holding agency environment, how do you go about building your custom allowlists?