Closest I'm not sure, but the best by far is Cafe de Vilis just off South Rd Mile End.
Wouldn't it be great if there was a pie floater cart in every suburb!! Unfortunately that's not the case.
Luckily Largs Bay is a stone's throw from Semaphore Road - some amazing bakeries on Semaphore Road, great cafes and restaurants. You're bound to find a pie floater at one of the local pubs too!
Best of luck, let us know when you find a decent pie floater so I can enjoy one also!