it seems due to guitar fashions, easier to find 5 and 7 string bridges these days.. but thorough internet search and ebay should turn up something eventually.
Some older style bridges have multiple grooves cut into the string block that would provide compromised intonation spacings for a double string course.. eg. i found a vintage 60's new old stock Baldwin archtop bass bridge which i intend using as a tenor electri mandola bridge when i find a suitable tailpiece requiring least amount of modification..