Why does it have to be a D? Do you absolutely have to have an incredibly low D? Like Jolly said, not unless you buy a bass. You tune that low B string down and it's going to be drooping like a broken string dangling from the machine head. Furthermore, if it's a cheaper 7-stringer like you said, then what's the probability that it's going to hold that tuning very well let alone sound good. And don't tune the B string up either, that's just damaging a string forcing it to do something the next string over can do without the problem. By all means though, try downtuning it and see how you like it, but what you see is what you get. They don't make strings that avoid being tuned up so in answer to your question, no there isn't a string that can help you. Even the heavier guages have their limits.