This website is by far the easiest place to find a Thai girlfriend or get laid without even having to leave your laptop.This is talking from my own personal experience and 8 western friends who all have found a Thai Girlfriend using this site.
Therefore unless you speak Thai, your going to have some trouble getting a Sideline Girl.
For foreign customers the girls typically troll dating sites that lots of foreign men are on, though your likely to get people who are just trying to milk you of money, if you want some good time with a girl for a few times you need to go to the dek sidleine sites that are in Thai language.
There are many websites on the Internet mostly notice boards, BBS, and forums where Thai girls make a new thread about where they are the price and services they offer and then guys reply back to the forum or otherwise email call directly.
I think some of the Sideline girls are no longer doing this for a side gig but more of a full time job, many of them you'll see pictures of them with their i-phones and fancy clothes, for some it's a necessity now because they are used to the high life.
The number one sure way to find a Thai girlfriend sometimes before even landing in Thailand is to visit the Thai Friendly dating site.