Those are hard to find i'vee been searching non stop for the past 3-4 months and cant find'em, the few that I do find Used they want an arm and a leg for them. But here is what I found, GB Cars http://www.gbcarparts.com/products.asp?cat=54
they carry 175/50 13 at the most reasonable prices I have found starting about $111.00 per tire. There was a guy selling a full set of NOS BFGoodridge T/A's 215/50 R13 on ebay, hey had it at buy know for $600.00! the actual bid went up to $420.00, hes was charging $75.00 for shipping. Kumo makes a 215 50 13 V710 RACE TIRE that is DOT Approved that is still available, the cheapest I've seen it is on ebay $115.99 ea. here http://www.ebay.com/itm/215-50-13-KUMHO ... b9&vxp=mtr
Other than that, there is nothing! I even tried having some shipped from the UK and it is extreemly expensive to have'em shipped to the U.S. so not really a viable option unless you want to spend $800 + on a set of tires.
..and that where my search has taken me... I am just moving up to a 15" and do a 195/45 15 the smalles of the 15" tires, has just about the same overall diameter as a 215/50 13.
hope this helps ...