GIMP will work just fine (I just made some) and not to mention free.
Here is a simple mesh brush, place it in the Brushes folder. The built in resize can make it look trippy...
@FeynmanTechnologies (View Post)
Well first you can create the pattern you want in black and white. White is where it draws elements. (I actually made mine in mspaint and opened in GIMP to convert).
Then in the top menu do image->mode->greyscale. To save correctly, do File->Export...->Choose 'Raw Image Data' from the list ->Name the file something with a .ptb extension.
Place it in the folder and hopefully TPT will let you select it by tabbing.