I agree with Hopkins on this one. I'd go with poplar if it is cheap and available locally, especially if you're doing a solid color finish (although you can get away with a burst if you find a nice enough piece).
As for Fender switching over to using alder instead of poplar etc. , Leo was a business man and used whatever he had on hand, nothing went to waste. That's why in the early days, Fender used pine and other cheaper woods for guitar bodies. They were cheap, he had a good supply, and he made more money.
Now that alder (and ash, and maple) has become the norm for Fender instruments, those are the woods that everybody wants. If they mysteriously switched over to basswood or poplar, or even pine again, there would likely be a slump in sales because everyone is so used to their alder instruments. I honestly don't care what the instrument is made of, I'm not in the tonewood camp. I've fallen in love with the sound of many different basses, and my main gigging bass is a plywood body precision bass.
I'd try the poplar, as I'm sure it will end up being a nice bass, and every bit as good as an alder bass.
-Ron