I don't know where this idea that Poplar is Aspen comes from???Depends if you're bothered about getting real poplar or not
There's plenty of yellow poplar (tulipwood) around (longbowshop do them I think, as well as Richard Head and some eBay shops) but as far as I'm aware it's only Lee Ankers who's making shafts from real poplar (aspen) now, and he only started this year. He'll have them readily available in a few weeks in all sizes.
If you want traditional medieval shafts that are similar to medieval poplar try willow, alder or lime wood shafts. In 3/8 sizes they'll be very similar in mass to aspen. The tulipwood sold as "poplar" in archery shops is usually quite a bit heavier than true poplar.