Ordering them from Petco?! Ah man thank you for providing me with a good laugh this morning.
Buccochromis rhoadesii aka Yellow Lepturus aren't terribly common in the hobby. The best places I can recommend to get them from are:
I know neither of them have them in stock currently but I would email them both and see what future availability looks like.
Also I hope you have a large tank ( 180 gallon +) available for these fish if you are serious about obtaining some because they grow to be 16" long.
Be prepared to spend some $$ too depending on the size of these fish they can be pretty spendy plus shipping will run $30 + . Big Malawi haps are worth every penny though in my experience, I commend you for wanting to stock something outside the norm.