I find these surprisingly hard to find. I have purchased three sets recently. Here is where I got them:

for the P-40B I got them from eBay. they were made by this company Montex. It is one of their maxi mask sets - Since it is a Polish company, you can't buy direct. what I do is check Sprue Brother, of Scalemates . I search for the item on there, and they have a list of places to buy them.

For a F4F-3 early - from kitmasx. they are Canadian and you can buy direct.

I also am trying some from from Kitlinx. This is a new vendor for me. I just found them from scalemates.com

What I honestly do is google around until I find an appropriate product and then try to find a place to buy it, usually through scalemates.