Both of these two sentences could be correct, depending on what you want to say, but they have somewhat different meanings.
I would like to know if you will stock this record.
This is asking if they will stock it in the future, but isn't asking anything about right now. This might make sense if, for example, it's a new record that hasn't come out yet, and you want to know whether, when it is released, they will stock it at that time, or perhaps it's a record you already know they don't stock currently, but you want to know/ask if they might change that in the future.
I would like to know if you stock this record.
This is asking if they currently (or usually) stock the record. If the answer is "yes", you would then reasonably expect that they could sell the record to you right now (or if they're currently out, they will have it back in stock soon).