Get training on procurement process through whatever level of government you are with (e.g. US Federal, Ontario provincial, etc.). Then you will know what purchasing options are available. The problem is that many departments and agencies have cut their procurement & supply staff, and those remaining tend to be at best amateurs in their knowledge about IT purchases, and most IT departments staff avoid dull training like procurement, and thus their purchases are done in a very ad-hoc manner, and often reflect personal bias (like only buying from Dell).
In the USA, at the federal level the government U.S. General Services Administration runs a program GSA Advantage.
In Canada the federal government Public Works and Government Services Canada (PWGSC) runs a Standing Offer Index including a specific one for microcomputers. For generic office PCs, these are starting points that are "pre-approved" purchases so you don't need to create and advertise and evaluate and have contested a RFQ (Request For Quotes), and as well the purchasers can also do LPO (local purchase offers) or RVD (Request Volume Discount) and likely other techniques I don't know of in particular if the Index is out of date, or a large volume purchase may warrant a discount.
Also learn and understand TCO - Total Cost of Ownership, that is the overall cost of the purchase (i.e. a PC in this case) including repairs / replacement components, down time due to failure - including the cost of have the employee unable to fully function at their job, plus cost in time & labour of IT staff for deployment (roll-out) including disk imaging (by vendor or IT), as well as overall satisfaction of the agency with the IT department's perceived value. This is standard IT management (CIO and below) knowledge, so talk to an intelligent IT manager for any help you need in this regard.
Unless you have scores of idle IT staff, assembling 1000 PCs is a lot of labour and time. And that's assuming you pick a hardware configuration that works, is stable, and is reliable. Most IT departments are staffed with strictly software oriented system administrators as their rank and file, with reasonably few if any computer hardware technicians and tools. And "at-home builders" experience don't cut it in a professional setting; it's a starting point for new hires, not a substitute for fully trained technicians. I'm sure that will cause flames, but I don't care, and to wit, I'm not a computer hardware technician, but I have respect for the few I have worked with, they were excellent at their job, and knew which end of the soldering iron to hold when making custom serial cables.