Personally I have no bias to which is the best service to write for. I have researched more on iwriter.com which is why I use them more as examples.
My recommendation here is to look through this list and select ones which you like the look of and then do your own due diligence...
So without further ado...
FOUR Writing Service Providers You Can Start Working For Today
iWriter.com (International) pays through Paypal.
Hirewriters.com (limited countries) pays through Paypal
Textbroker.com (limited countries) pays through Paypal
Freelancercareers.com (not specified) pays through Paypal.
All of the above pay weekly through Paypal provided your have earned over $10.
Freelancer Sites Where You Can Sell Your Writing Services
Elance.com now Upwork.com (International) payed via invoice process less 10% Upwork.com fee.
Fiverr.com average $5 writing gig gets 1 x 500 word article
Revenue Share Sites Where You Can Get Paid For Your Writing
Hubpages.com write articles get approved and earn a share of advertising earnings from your content.
Infobarrel.com Click the url to go to the writing for Infobarrel page.
Two reasons I Don't recommend this route...
1. Your very badly paid (click through and read some of the forum posts).
2. These sites are out of your control, therefore so is your content and your potential earnings. A risk not worth taking in my opinion.