I would like to use a sentence like
It should be being sold at somewhere for an affordable price.
Would it be syntactically correct ?
Yes, but it seems better without the first at.
It should be being sold somewhere for an affordable price.
This is syntactically correct, but it's certainly an uncommon way to get the thought across.
You can, of course, just say:
It should be for sale somewhere at an affordable price.
Note, you can also use the present perfect passive with a modal:
It should have been being sold somewhere at an affordable place, that way I could have bought it when I needed it.
but there are less convoluted ways to express the same thought. Note also that some native speakers may find either your construction or mine marginally correct, and some may find it to be of doubtful usage, or say it is not syntactically correct. It really does come down to what sounds right to the individual, and in the individual sentences, in cases like these.