Welcome to the forums!
Unfortunately, we don't sell the G20AJ 180w adapter in the U.S. as far as I know, but many of our notebook models like the G750JW and G750JX use a similar 180W adapter which you can use instead although the tip is curved or 90 degrees rather than 180 degrees if it matters which makes it a little bit tougher to unplug from the G20AJ, but shouldn't be too big of a deal.
If you want to hunt down the notebook 180w adapter, it's sold through distribution under our sales pn 90XB00EN-MPW010 (this includes the retail packaging/box and we use a different pn in repair which doesn't include the retail box if you happen to run across one) and you can check our U.S. 'where to buy site' below to see if any of those resellers carry it or can order it. Just be aware distributors may only sell to resellers, so you probably want to avoid them unless you have a resellers license.
http://promos.asus.com/us/where_to_buy/?_ga=2.136041137.101370145.1511809662-1533455531.1449869626
Hope this helps. Thanks.