Dromarti make a slotted cleat that will fit a conventional 3-hole road sole - but they're about £16/pr.
There are the Dromarti shoes made by Maressi but pretty pricey too.
There are usually lots of old shoes on ebay
Vittoria have a shoe called the 1976 Classic - Brick Lane Bikes in London can order them or you could try ebay.it. The do different models - some with a modern carbon road sole or a rubber / SPD sole. I quite fancy a pair of the latter for L'Eroica and I can use them for the commute with SPDs.
When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.