Welcome in the wonderful world of graphs +Mike Atherton
, a world in which almost nothing walks a straight path. :)
Now if you're able to keep things simple by modeling your graphs as if they're tree-models than things should be OK.
Though it doesn't matter whether a graph is or isn't a tree. Main difference being a cross referencing graph can be harder to build and/or query, but hey, that's the nature of the beast. In return cross referencing graphs often also tend to contain much more valuable data.
If you're contemplating cross referencing graphs than things tends to become complex easily. So be diligent, accurate and patient. Rushing things normally won't help you out in the long run.
Now whether or not you have a single or multiple domain situation doesn't really matter as you can perfectly well makes semantic statements about entities smeared out over multiple domains as well as those contained within a single domain.
Now if I try to build a model the way I understood your description I get something like this:
(refers to the product being offered by the organization)
And then add as many 'availableAtOrFrom' relations as you need to define.