You are using an unconentional approach. Normally you would load all the balances into a single table and use the date to select the balances to use in an expression. What you are trying makes this quite a lot more complicated.
I would suggest that you consider adapting you model in this way. But if that is not possible, or you have other reasons for using your approach, to get the list of tables, you can use the $Table system field.
Hope that helps