Hi,
In case you don’t get a better answer, here’s a link to all the [https://www.asusparts.eu/en/partfinder/Asus/LCD%20Monitor/V%20Series/VC239H|parts] for your monitor. (If the information isn’t there on the webpage, just select ''Asus - LCD Monitor - V series - VC239H'' in boxes 1-4 and then select the “hinge” checkbox)
I don’t know your monitor but maybe the washers are part of the STAND-HINGE ASS'Y Part No. : 13010-01820900.
It may be that they are not available as a separate item as there is no mention of them in the parts list.
You could always try contacting the supplier and ask if the washers are available separately or not or if there are any installation instructions showing how they are fitted, (e.g. if they are a part of the hinge assembly perhaps there are installation instructions that come with it unless it comes pre-assembled of course.