Have you tried the Parts-People web page here? They do offer a genuine Dell OEM 240W adapter.
Hi, I have a G15 5511 laptop, which came with a 240W adapter/charging cable. It suddenly stopped charging (the light on the cable doesn't turn on) so presumably I need a new one The only one available on the website that shows as compatible is 180W, will that actually be sufficient to power my laptop? TIA 
G15 5511 240w adapter died, only 180w available?