Lon put me onto this.  Apparently, the main ingredient of Frog's Fanny is hydrophobic fumed silica.  I'm sure they have some other secret sauce mixed in there as well, but the fumed silica looks like it will do the job just as well. 

I tried it on some dry flies and did not detect any difference in performance.  Soooo... the price is hugely different.  You can buy a quart of fumed silica for $4.50 vs a very small bottle of Fanny for about the same price.  Unless you are obsessive about whatever else the frog guys mix in, just get the generic equivalent.

Now, if you do this, you will face the challenge of handling it.  This stuff is light!  You will not be able to spoon it into a bottle and, even then, you need a small brush to apply it to your fly.  The solution is to buy the Frog Fanny and keep the bottle.  Then, courtesy of Lon, get a turkey baster and use it to suck up the silica and squirt it into the empty bottle.  Be very careful when you do this.  Just like on the stream, you do NOT want to breathe this stuff as it is toxic.  Once you reload the bottle, you are good to go.

Go to this website and buy their "fumed silica".  It will work just fine.

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This is the stuff.  I got a lifetime supply it was so cheap.

Put a strong rubber band on the bag to keep the silica in

The rubber band allows you to insert the baster and suck in the silica without opening the bag - keeping it out of the air and you out of danger.  Be careful!

Final step... gently squirt it into the empty Frog Fanny bottle.