First I'd measure the arbor to see if it is imperial or metric. Then I'd go on Ebay and look for side and face cutters. A selection 3-4" diameter and from 1/4" to 1" wide would be ideal. It is possible to buy S&F cutters new, but they're very expensive and pretty much obsolete in industry. I've bought almost all my horizontal milling cutters secondhand, or new old stock, on Ebay. As with all items on Ebay these days there are a lot of fantasists selling cutters for inflated prices, but there are still bargains to be had. I wouldn't pay more than £10 for a cutter as listed above. Look carefully at any picture for worn, broken or damaged teeth. If present, move on and buy something else.
Andrew