Venue sourcing is the process of finding and comparing venues against an event's dates, capacity, layout and budget before putting one under contract.
Writing a specification, sending it to candidate venues, comparing what comes back, visiting the shortlist, and negotiating. The specification is the part that gets rushed and the part that determines everything else: dates, flexibility on dates, headcount, the largest single room you need, breakout count, catering requirements and budget ceiling.
Venue pricing moves with demand. A conference that can shift by a week, or move from a Tuesday to a Thursday start, has a materially different negotiating position than one locked to a fixed date. If your dates genuinely cannot move, say so early rather than discovering it halfway through negotiation.
Ceiling height and sightlines in the main room, power and rigging, loading dock access, the real walking distance between rooms rather than the one on the floor plan, wifi capacity under load, and where registration can physically go without blocking the entrance. Photographs flatter venues. Walk the attendee's route yourself.
Service charge and tax on catering, AV brought in by the house provider, internet above the basic tier, and overtime on labor if your schedule runs past the contracted window. Ask for a sample bill from a comparable event rather than a rate card.