Where can I get a replacement cushion for a flare back vanity chair?

What type of chair? The answer may depend on the type of chair. For dining table chairs you can add tie straps or straps with Velcro.
For upholstered chairs i use loops with D rings sewn into the back of the cushions that can be clipped to the frame of the chair.
Velcro can work as well with upholstered chairs but placement can be a problem if you are able to flip the cushions. The Velcro need to be on the back of the cushions so it's not visible.
Hope this helps!

get strong help

Here you go:
http://www.ebay.co.uk/itm/NEW-Prima-Pappa-Highchair-Cover-In-Medium-Brown-WIPEABLE-/231189872271?pt=Feeding&hash=item35d3fd7e8f

look at wayfair .com for replacement items for your chair

I looked up the model number for your Safari Sun pattern and see it is 1758122. When I put that in on Graco website with your mfg date it comes up with replacement parts. Graco Replacement Parts The cushion is not listed, however if you search your model of high chair in Google, "Graco Meal Time High Chair" you come up with Amazon listings and can see the colors available. When you find a suitable color, go back to Graco website and enter the model number listed on Amazon and your mfg date and you will see if you can buy a cushion in other colors. If still no luck, call Customer Service for more assistance.
Graco Customer Support is available Monday through Friday, 8am to 5pm EST. 800-345-4109

Yes! Can find replacement seats online or at local shops

There are many sites with a variety of cushions to choose from. I recommend a weather-resistant cushion like this one - http://www.plowhearth.com/weather-resistant-outdoor-classic-adirondack-cushions_p1469.html

You can also make your own cushions if you would like. There is a great article and tutorial on making your own Adirondack cushions. It could be a fun project.

http://www.ehow.com/how_4898460_make-adirondack-chair-cushion.html

Have you tried cushions direct? They have a good selection of rocking chair cushions http://www.cushionsdirect.net/rocking-chair-cushions.html

Here are a list of current cushions available for Perego Prima Pappa models. The correct cushion depends on the model you own.

http://www.momsjustknow.com/replacement-coverscushions.html

U can go to ebay or look on peg peregos website they have all kinds of things to purchase!