The micro-purchase threshold is the dollar amount below which agencies can make purchases without soliciting competitive quotations, and it is set by FAR 2.101.
For most supplies, the micro-purchase threshold is set at a specific amount. For construction services subject to the Davis-Bacon wage requirements, it is a lower amount, and for services subject to the Service Contract Labor Standards, it is also a different, lower amount (FAR 2.101).
You will not see formal notices for micro-purchases on SAM.gov because proposed contract actions expected to exceed a higher threshold must be synopsized there, not micro-purchases (FAR 5.101(a)(1)). Purchase card holders, also known as Government Purchase Card (GPC) holders, are typically individual employees authorized to make these small buys directly. There is no central, publicly available list of purchase card holders for an installation. Your best approach is to identify the various units on the base that might need your industrial supplies and then reach out to their supply or administrative offices directly. You can introduce your business and provide a capability statement. Many installations also have small business offices or APEX Accelerators that can help you understand local purchasing practices and connect with relevant personnel.