[link|http://www.chipcenter.com/circuitcellar/december00/c1200rp60.htm;$sessionid$WQVY3WQAAFXIIQSNDISCFEQ|Some general relay links]
A number of places [link|http://www.measurementcomputing.com/cbicatalog/directory.asp?dept_id=25"|including Measurement Computing] make relay racks; many are compatible with Opto 22's racks. You might have to search harder to find double throw.
Another popular way to go is DIN-rail mount relays, either embedded in slim-line modules (e.g. Phoenix Contact & competitors) or normal relays with DIN-rail mounts, with my old standby being [link|http://www.idec.com/usa/index.html|IDEC] and new favorite [link|http://www.findernet.com/index.php.en?country=US&gruppo=gruppo3|Finder].
If you need more help, try calling some of relay companies or their local reps or distributors.
Tony