Either your military ID (CAC) and/or your LES or pay statement should be evidence enough of your active duty status. If you are no longer receiving paper pay statements, you should be able to access it electronically via AKO. A copy may also be available from Army HRC via the MyRecord Portal.
If any of that is insufficient, then go here: https://www.dmdc.osd.mil/appj/scra/single_record.xhtml and enter your name and DOB to obtain a certificate of your duty status. The Defense Manpower Data Center SCRA web site was designed for the specific purpose of validating an individual's active duty status. Per section 582 of the Service member's Civil Relief Act (SCRA), the certificate from the DMDC web site is prima facie evidence of active duty military service.
Also, be sure you are dealing with a reputable lender/broker as they should already know how and where to get the information that validates your eligibility for a VA loan.
I would be leary of working with any institution that isn't able to tell you exactly what they need from you AND where to get it re VA loans.
Scroll about a 1/3 of the way down the page: https://www.hrc.army.mil/Enlisted/Enlisted%20Reserve%20Soldier%20Services%20-PAB%20Functions-
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Your other option could be a copy of your orders or a letter from your commander.
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