There are three third-party vendors that we know of which provide what you are looking for. Here is what we know about them.
Attachmate's EXTRA! for Macintosh 2.0 is a TN3270 emulator that lets Macintosh users connect to mainframes over NetWare for SAA as well as over TCP/IP, FTP file transfer to IBM hosts. Also a new 3278 host-printer emulation lets you preview, format, and print terminal documents to AppleTalk printers.
5 PM TermOffice 3.0 from About Software provide a wide range of terminal emulations, including those for IBM mainframes; AS/400 midrange computers, and VAX, UNIX, Unisys, and Honeywell/Bull hosts.
The following information was taken from an AppleLink article in the "News" section of the "Third-Party Press Releases" folder. The article is called "Update to TCP/Connect: IBM TN3270 Terminal Emulation".
InterCon's TCP/Connect gives users access to the power of the Department of Defense networking standard TCP/IP. TCP/IP was developed so that computers of different types could communicate over a mixed-media network. TCP/IP offers a standard way to transfer files, emulate terminals, and other functions between dissimilar computers and operating systems.
Apple has announced support for TCP/IP with the introduction of MacTCP (a set of TCP/IP drivers for the Macintosh). InterCon supports MacTCP. IBM 3278 emulation is included in the TCP/Connect-Extended and in the TCP/Connect-3270 software packages. TCP/Connect features include Telnet, FTP, and varied hardware support:
Telnet
Telnet is an implementation of the standard, virtual-terminal protocol through which you log onto any Telnet-supporting host. Features included in TCP/Connect are:
Tektronix(TM) 4014 emulation
IBM 3278 emulation through TN3270 Protocol
DEC VT102 emulation
DEC VT240/241 graphics terminal emulation
Copy of graphics from any of the graphics terminals
Copy/Paste of text between sessions and other applications
Up to 20 simultaneous sessions
Scroll-back within the terminal window
User-defined macro keys
FTP
FTP, the File Transfer Protocol, lets you transfer files to and from remote hosts using a Macintosh. The remote host must have an FTP server in operation. ASCII, binary, and MacBinary transfers are supported.
Supported Hardware
Hardware supported by TCP/Connect include:
Macintosh Plus, SE, SE/30, II, IIx, and IIcx
Kinetics EtherSC(TM), EtherSE(TM), and EtherPort II
Dove FastNet II(TM), FastNet III(TM), Marathon LAN 020
3Com EtherLink II(TM), EtherLink/SE
Everex
Apple EtherTalk any EtherTalk compatible device
Including any LocalTalk to Ethernet gateways, like the GatorBox by Cayman Systems, and Kinetics FastPath devices.
The Tech Info Library article titled "Locating Vendor Information" can help you search for a particular vendor's address and phone number.