Never heard of SLP having their own pulleys, but they do have a "package" that they put together. Pretty much the only thing I know of that SLP makes for mustangs is exhaust. Mind you, some hate it some love it, but the quality of their products is outstanding. If they do make their own pulleys by chance, more than likely they are a good set.
Yes they do make there on set. Just look at there website slpshop.com and they are on there. I was looking on there the other day and found them and they look just like the Steeda ones.