Orgillion Horror is a nice series of adventures, but if you already started in Jaiman and really want to play a longer campaign, I think you should consider getting the Jaiman book. It gives a good overview over the continent and the political situation, and includes the Legacy of the Sea Drake Campaign. The adventure sites are pretty well described, but you have to work up a bit on the story in between. There's also a longer dungeon/tomb adventure (site) included, an adventure which can serve as an introduction to the other campaign. Its all RM 2nd Edition.
"Cloudlords of Tanara" is a very old module set in the West of Jaiman. It has some nice adventure ideas, but some of the content needs to be reworked since it was published before even the first Master Atlas. You can either meddle into the politically unstable situation or try to hunt down an evil elf with a very nasty sword: The Implementor.
If you start from Norek, you can also try "Demons of the Burning Night". Its not really a campaign, but a large adventure site on an island south of Jaiman. A lot of very evil creatures lurk there.
In the far north of Jaiman, you can try "Quellbourne, Land of the Silver Mist", not really a campaign, but a good setting for playing one. There's also an old book called "The world of Vog Mur" which is set in the vicinity, but I never read it. The "Stormriders" Novel is closely linked to the Quellbourne setting.
If you want to run a long term campaign, but don't care about Jaiman, you should perhaps try "Curse of Kabis". It hasn't got the usual Shadow World, but RMSS layout, but don't be fooled, it's set in SW. I never played that one and only know that it's centered around a pretty hefty demon (Kabis), but I heard that it's a very good campaign.
As to buying the stuff check antiquarian databases, amazon marketplace, ebay.
