You cannot get nor create models for Phoenix unfortunately. You can however download and or edit new skins for existing models as I do. You need to download Phoenix creator and have a basic understanding of importing/exporting BMP files for using third party skins in the sim. And if you want to make them for yourself, you will also need programs like photoshop or corel draw to bounce back and forth from.
Its really an irritation and a shame that no one has figured out how, or the fact phoenix won't setup a model creator for scratch designers to tinker with since they take an eon to add anything new and worthy to keep em on the top ranking.
Hopefully there will be something for it in the future, but I wouldn't hold my breath over it.
Josh