If you didn't lock down a USAR slot at LDAC, you can still get a vacancy hold from a local unit.
The easiest way to do this is to get on the HRC website (hrc.army.mil) and login to the user portal. If you are a progression cadet and don't have an HRC portal access, have one of your prior service cadets or one of your cadre use theirs. Once logged in, there will be several options to click - click the button called "Tools" and in the subsequent menu, there is a tool that can find vacancies.
Using that tool in the HRC portal, you can search by MOS code, state and grade. They'll even give you the unit name, the POC # for their personnel section and the paragraph and line number of the position. Just FYI, if you're looking for a position, look for the 1LT positions. The USAR doesn't tend to have 2LT slots, mostly because they have greater rank flexibility with 1LT slots. You will be able to take any of the 1LT slots in USAR as a 2LT.