The lamps used in all Rubidium standards are custom-made. There are no replacements for old units other than by disassembling another of the same model. Newer units might be repairable by the manufacturer, but I'd hate to think of what it would cost. If the original lamp isn't broken, you can try to rejuvenate it as described here: http://www.n4iqt.com/efratom8084008/eeratom_model_frs_lamp_assembly_repair.pdf . This appears to be a newer, expanded version of the article on www.radcomms.net.
Are you really sure that the original lamp is the problem? Those lamps last a very long time!