Where Do I Find The Update URL For Google Chrome Extensions?

I can't help you with group-policy, but each extension includes its update URL in manifest.json.

So, for the current version of adblock (id: gighmmpiobklfepjocnamgkkbiglidom):

%USERPROFILE%\AppData\Local\Google\Chrome\User Data\Default\Extensions\gighmmpiobklfepjocnamgkkbiglidom\2.5.14_0\manifest.json

Contains:

"update_url": "http://clients2.google.com/service/update2/crx"

The extension will query that URL for updates, as per the documentation.

We can therefore construct a URL that will return the update XML from the above URL (just change the ID as needed) - for adblock:

http://clients2.google.com/service/update2/crx?response=updatecheck&x=id%3Dgighmmpiobklfepjocnamgkkbiglidom%26uc

The XML that is returned reads:

<?xml version="1.0" encoding="UTF-8"?>
<gupdate xmlns="http://www.google.com/update2/response" protocol="2.0" server="prod">
    <daystart elapsed_seconds="49387"/>
    <app appid="gighmmpiobklfepjocnamgkkbiglidom" status="ok">
        <updatecheck codebase="http://clients2.googleusercontent.com/crx/download/OAAAAFpzXu4buuGNADfzIKiz34SLARZdBLiXQ2zo50sAlzoBpEz77foH-XT3yHpPureXtHcQSYU2z4ZNstiuKJi-LD8AxlKa5VgufvySdIb5b9U333P0upRk1YPb/extension_2_5_14.crx" hash="" size="529317" status="ok" version="2.5.14"/>
    </app>
</gupdate>

We are interested in the codebase attribute of updatecheck, which provides us the direct URL to the latest CRX file.

The accepted answer is right. To save you the hassle of searching on Linux, the folder is at

~/.config/chromium/Default/Extensions/