Can you buy gypsum anywhere outside of home-brew sites or stores? Also, is there a standard amount used in extract batches? Thanks!
Gypsum is Calcium sulfate and you can get it at most health food stores and even drug stores. I’ve seen it at Asian Food Markets as well.
I don’t know if you can get it elsewhere. There is not standard amount. it has to do with what you are trying to accomplish combined with what you are starting with. with extract that’s particularly problematic because you don’t know what the maltster already put in the extract. if you begin with distilled water then that at least is a blank slate.
best thing to do it brew your recipe and taste it. does it seem like it would benefit from a more crisp, dry seeming finish? maybe a bit more pop in the hops? then dissolve a few grams of gypsum in an ounce of beer and use that to dose another sample until it tastes the way you want it to. you can then extrapolate how much you would want to add to the whole batch to get the same effect.
you can then include that in your recipe and the next time you brew it, assuming everything else is the same, you can add it in the boil kettle.
+1 - Less is more with extract and gypsum. Start small and find the amount you like, and you can scale up from there.
Unless you feel you’re short of CA for the fermentation I would only use gypsum in the final beer to taste.
Yes, you can buy gypsum elsewhere. Take a chunk of sheet rock, peel off the paper, scrape it a bit to get rid of any residue on the surface, and then crush it up. Viola! Gypsum.
Might not be food grade. 
Malt extract will contains some minerals that were present in the companies brewing water. Your own water may also contain some, so I wouldn’t assume you need it. Since sulfate is for flavor you should brew without it and dose a glass to see if it improves it.
I do use gypsum in extract beers, but only in very specific conditions. I wouldn’t add it to any recipe unless you’ve brewed it first. Between the extract and your brewing water, it’s hard to tell what kind of sulfate level you have in your finished beer. But if you find that your hoppy beers are missing some dryness to the finish, then you can certainly experiment with gypsum. About 1 tsp in a 5 gallon batch is a good starting point for an IPA.
I use 1/2 tsp in a split 5 gallon extract batch occasionally to check out new hops, and the beer is never ‘minerally’. I always use Briess Pils DME for consistency.
EDIT - I found some very old brewing notes on an all-extract IPA where I used 1 tsp of gypsum for 5 gallons, and it worked well.
Make sure it is not from China. Just saying.
I would use food grade.