You're probably thinking of a
Localtalk to Ethernet bridge. The linked Wikipedia article has the names of some of the more common ones. They come in several different flavors; the "dumb" ones only watch the Ethernet port for Ethertalk DDP packets and bridge those over to the Localtalk side (and vice-versa), while some of the more advanced ones have built-in routers able to handle talks like TCP-over-Appletalk encapsulation.
The "dumb" ones were often sold as a solution to get Appletalk-only-equipped printers onto an Ethernet network, and therefore are sometimes called "printer bridges", but they work just fine for data. I have one and it works fine for giving a Mac (or an Apple IIgs) access to a Netatalk 2.2.x server, and you can even bridge TCP/IP over it using a solution like macipgw.
(Mentioning the latter just to make clear that if you want the bridge for Internet access, like what you're running SLIP for, you either need one of those smart "router" bridges or you'll still need a computer running bridge software on your network somewhere, the dumb bridge alone can't give you TCP/IP access.)