To identify federal agency spending patterns for specific services using FPDS and USAspending.gov, focus your search on the NAICS code and then apply additional filters like the Product Service Code (PSC), agency, and contract type. I've used these sites to see who's buying janitorial services, not IT, but the approach is the same.
Start by searching in USAspending.gov with your NAICS code. The results will show you prime recipients and award amounts. Then, filter by agency to see which ones are spending the most. You can also narrow down by contract type, like firm fixed price or time-and-materials, if that matters for your service. FPDS provides more detailed information on individual contract actions, including specifics on the solicitation, awards, and modifications. You can use it to see if an agency is consistently awarding contracts for a particular service, and to whom. Agencies must report every contract action over the micro-purchase threshold to FPDS, so the data is pretty comprehensive. This lets you see if a contract is a recompete or a new requirement. I prefer county and school work because it's less complex than federal procurement.