chornedsnorkack
You are correct, there are airports where you will not find an alternate close enough to use as a take-off alternate. But, when this is the case, then I think you'll find that there are seperate rules in force with regards to wx minima for take-off, such that take-off is not allowed if the weather is forecast to go below your OEI minima in case of a return.
This being the case then you would only depart in conditions that did not require a take-off alternate.
The above is just my bit of logic and digging back into the recesses of my mind, as we currently don't operate to any isolated destinations.
Regards Expat