Creating your own is much easier to tile than a photograph. If you have PhotoShop, you can use the "offset" as Indi has stated. (it'll save a bit of manual labor)
Rendered clouds dont seem to need any alterations after offsetting them.
1. Create a basic image you want to tile. (random blocks/foliage/etc)
2. Filter -> Other -> Offset
3. Make the 2 values half of your image's sizes (50%), and selecte "wrap around".
4. Fix any gaps or mismatched pixels.
5. All done!
That's essentially what I did, only I was doing the offsetting manually.