How should I access multiple ItemsSources from filtered collections when the objects get unwieldy?

Question

I have the equivalent of a clothing store. I put in standing orders for clothes to be delivered each month, but some things can be flexible each month - for instance, I know that I am more likely to sell black clothes during the winter and tighter sizes during the summer.

There are some elements of the order which don't change, and some which do. I have the overall order in a panel and the changeable items displayed in tabs by month. Some of the ItemsSources within the ComboBoxes for each month depend on values for the order; some depend on values for that month.

For reasons of performance and because the classes are getting large and unwieldy I really want to be able to get two different DataContexts for each month as follows, but obviously I can't:

---------------------------------------------------------------
|     Order                                                   |
|     OrderItemsSources                                       |
|-------------------------------------------------------------|
|                         |                          |
|MonthlyOrder[0]          | MonthlyOrder[1]          | etc.
|MonthItemsSources[0]     | MonthItemsSources[1]     | etc.
|                         |                          |
-------------------------------------------------------------

Those are tabs at the bottom; only one is visible at a time. Each tab contains about 20 fields drawn from the MonthlyOrder with its ItemsSources as potential values.

At the moment I can only access those MonthItemsSources through the actual MonthlyOrder. My MonthlyOrderViewModel wraps both the MonthlyOrder and the MonthItemsSources, which are the possible values to which the MonthlyOrder's fields can be set, given the other choices of fields already made.

---------------------------------------------------------------
|     Order                                                   |
|     OrderItemsSources                                       |
|-------------------------------------------------------------|
|                          |                            |
|MonthlyVM.Order[0]        | MonthlyVM.Order[1]         | etc
|MonthlyVM.ItemsSources[0] | MonthlyVM.ItemsSources[1]  | etc.
|                          |                            |
-------------------------------------------------------------

Is there any way to do separate the MonthlyOrder from its ItemsSources? Any way to pass an index into a binding, or a reference of some kind, so that I can get the tabs into the form at the top?