Outdoor fabric can be used to cover your cushions but oftentimes there is nothing affordable about outdoor fabric. It might be the best way to go because it will be the most waterproofed. However, I have found other alternatives to cover fabric on the cheap. What about using shower curtains for covering your outdoor cushions. Sometimes you can find a few of the same shower curtains at a low cost and use the fabric from that. The only concern I would have for that is the fact that shower curtains may not be thick enough to be used for seating. Another idea is to use regular cheap upholstery fabric that you find on discount and spray it waterproof. there is a way to turn regular fabric into outdoor fabric. Look at this spray product called Thompsons fabric seal... https://www.thompsonswaterseal.com/waterproofing-products/small-projects/waterseal-fabric-seal. You might do fine with spraying whatever fabric you have or buy with this waterproof spray. Only spray the already cut to size fabric. You will need at least a few heavy coats I would think to turn absorbing fabric into an outdoor fabric. Maybe get a thicker upholstery fabric that you spray for the cushions and use the shower curtain fabric as outdoor pillows. Just a thought but hope it all helps! Good luck!
Here's a tutorial on how to make a new cover: https://www.youtube.com/watch?v=eeSyPdHtmOY
The best fabric to use for outdoor patio cushions is fabric with a backing, and fabric that has a dense (this is different from thick) weave to it. A nylon or nylon/polyester blend will be more resistant to abrasion and stronger than a basic cotton, and therefore, should last longer.
Fabric labelled as 'outdoor' fabric is usually more expensive than similar materials that are not marked as such. This is because it has already been treated with water resistant spray. The high-end brand-name fabrics, such as Sunbrella use special fibres that are resistant to mildew, and special dyes resistant to fading. 'Outdoor fabric' also usually comes in wider widths than fashion fabric.
Only 100% vinyl will be completely water proof. However if sprayed with Scotchguard spray, most fabrics can become quite stain and water resistant.