I just went through this myself. Any 500A or 835 stock will fit on your 590A1. For the forend, you need one for the 7-5/8 action slide. Most of the ones you see for sale are for the longer set.
Havlinsales.com sells the wood furniture in the raw. Here's a pic from their website that will help you figure out what size you're looking at, if you come across a used set.
The top forearm is for the shorter 6-1/2in action slide; the bottom forearm is the one you want and for the longer 7-5/8in action slide.
Throughout the years, Mossberg has used different styles of checkering on their wood furniture. The corncob forend seems to be the most desirable and hardest to find. There's also a ribbed version and a checkered version. st1264 above posted the checkered, and Gronk posted the corncob. Here's a pic of the ribbed version:
For the stock, there are also several variations. There are a few types of checkered and also a non-checkered smooth version.
When I decided to put wood furniture on my 590A1, I first chose which model of 500 I wanted to emulate - the 500ATP. The correct wood is hard to find as a matched set, and when it does pop up for sale, it always goes for $100+. I ended up just buying an actual 500ATP and swapping the furniture.
Here's my 590A1:
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