Photoshop does perlin noise(render clouds) tilable if you use a power of 2 number. (256, 512, 1024, etc) This applies to the difference clouds as well. I use that a fair bit for distortion textures. I also do a lot of hand painted maps for distortion… just using photoshops tiling features. There are a number of free maps for things like curl noise around the net and occasional free apps as already posted above. Some even directly as web apps like this
Honestly it’s a great time for this kind of thing because there are so many resources both free and paid which will get you there and a few thousand tutorials on how to make your own.