Ouch...that is not what you wanted to hear!!
As for where to go, since you have to travel anyway, I would be SURE that the center you choose to be referred to does have expertise in treating sarcoma/carcinosarcoma in the bladder. I would contact the urology departments in each of these and ask that specific question. Others to consider, who do see rare conditions, are MD Anderson and Memorial Sloan Kettering. Just because someplace is well-known, does not mean that it has competence in everything.
Does your urologist have a specific physician he is referring you to? That would indicate that, indeed, he does know someone who is an expert in sarcoma/carcinosarcoma. If it is a general referral to "Mayo Clinic" I would wonder.
Best luck to you
Sara Anne