You can make your own using the logo generator and image magick. It's
At the logo generator choose gif as format.
In the text box type a single letter.
If you want an invisisble gif to be say 100x50 set the size at the logo generator at 110x50.
Set font size at ten.
Set the font to be aligned left and top.
Click black for text color, transparent for background.
Make the logo.
Cut and paste the url of the new "banner" and take it to Image Magick.
Now you will be cropping the letter from the gif.
Hit the transform at the top of page.
Next page put image size and +10 in the parameter box. Like this:
( you could put +100 and end up with an image 10x50)
Check "crop" then the transform button.
Next page should show you the transparent image (100x50) without the letter, then check output at top of page.
Next page check gif, single file, then the output button.
Next page click on image to see it on a page of it's own.
Use the go to key for the url.