This is a list of resources for those who need a gambeson, whether they wish to buy one or build one of their own.
Where to buy
When shopping for a gambeson, there are few things to keep in mind. The general rule of thumb is that if you are wearing flexible armor (mail, coat of plates, lamellar, brigandine, etc) you would want a thick gambeson. If you are wearing solid plate, you only need a thin gambeson. Your mileage may vary, of course.
These listings are arranged approximately in a rough order to represent the usefulness I found in these links, but again, your mailage may vary depending on your budget and need. I take no responsibility for your dealings with any of these merchants, feel free to discuss with folks on various message boards such as the Armour Archive and on Facebook before ordering.
For Reenactors and SCA
These producers and merchants make and sell gambesons to folks who intend to use them for armored combat with either blunted steel or rattan.
Sold for LARP
The producers and merchants here target their stuff for folks who do Live Action Role-Playin, usually involving fighting with padded boffers.
Sold as Costume
The merchants here sell stuff targeted at people who do not intend to wear it in combat. Some of the stuff is good enough for combat, but whether it is or isn't, I have included it here so that should you find something on your own, you can cross-reference it here and make your own decision.
These are a few reviews of gambesons for sale
These are websites that give instructions or advice on how to make a gambeson.