I believe you have ascertained that you can hold off. Same suit is fine for callbacks.
You can get some great ties on ebay for < $40, Canali, Zegna, purple label, whatever suits your fancy.
I went into my interviews with the attitude that, "If I sit at my desk all day getting high-quality work done, on-time, then you look good to the client."
Good luck and have fun meeting your colleagues.