Bahtinov masks are available from various manufacturers and retailers. You may also generate your own mask using the astrojargon Bahtinov Focusing Mask Generator. One advantage to using the generator is that you can experiment with the parameters to tweak the performance of the mask.
I generated a mask for my ST80 using the default parameters, printed it on card stock, and cut out the slots with a hobby knife:
|Bahtinov Mask printed on card stock.|
The image below was taken while focusing on Sirius:
|Sirius through the Bahtinov Mask. At left, the image is out of focus. At right, the star is in focus.|
Despite the mask's ease of use, though, I still cannot see the monkey.