Forms :: One Form 2 Tables?

I have

5 Tables

[Estimate]
[Invoice]
[Clients]
[SalesPersonID]
[AccountTerms]

2 Forms

[Invoice]
[Estimate]

At the moment, what I'm trying to do is have a form setup as our Estimates currently in use. I've created a Form and arranged as required to make said paperwork everything so far fine.

Now I tried adding a Combobox to select a client from the [ClientsTable] which is fine, and it will even fill in some text boxes with the relevant information, again perfect.I cannot add a new Client to the [ClientsTable] from this Estimate form, and in turn can only save a client to the [EstimatesTable] if matching fields exist, if these fields are missing, it is then blank.

Surely duplicating the client repeatedly into the [EstimateTable] is bad practise, yet I cannot get it to work any other way in this regard. Even id I do this I'm still stuck with the Combobox only referencing clients in the [ClientsTable] which means there is plenty of room of incorrect Client information.Now I understand this in part, but what I need to be able to do is pull up existing Clients from [ClientsTable] and be able to add new ones from the [EstimateForm]

I really don't want to have multiple forms for all the different Tables as this is inefficient and my staff will never understand what there doing.So how can I add multiple Sources to a form? I'm looking into Subforms but this still seems inefficient instead of just linking to existing tables?