Depending on what size transparent result one wanted and what the intended purpose was, it would be theoretically possible to make an interneg using Portra 160 and then print on to the display-transparency RA4 material Fujiflex. That route would be using materials that are all in-date and available, as the older interneg and direct-copy films (perhaps what the OP was thinking of) aren't made any more, unless someone knows of a hidden manufacturer somewhere. Thirty years ago the lab I worked at would enlarge 35mm on to 4x5" interneg film, rather than contact-print, and then use RA4 from that in the usual way.