I was looking for some DT Swiss New Aero j-bend bladed spokes for a custom wheel build and was informed that these spokes are not for sale individually but only used in pre built wheels for clients such as Trek. So, sourcing these spokes took some digging, but I did find this website:

http://trekbicyclestores.com/

They had 2 of the sizes I needed but not the 3rd. I asked my local bike store who had an account through Trek and they were able to get the last size for me.

So, I guess you can potentially find out what prebuilt wheels out there use the spokes you want and try and find 'replacement' parts through their website.