How do I match an old molding pattern?
If you are serious about this project, you have two options:

1) Scour various retailers and specialty stores (and the internet?) for a moulding match at an appropriate price.

2) Take a sample of your moulding to a professional millwork shops and they can match it. 

Option 2 will likely be pricey depending on the complexity of the work piece. 
We had carved hardwood moulding in the house I grew up in and the mill wanted over $20 a linear foot to make it. 

It looks like yours can be made from foam injection, or simple routing... so it should be less expensive.

Another option, which may be cheaper but more work:
3) Remove all the moulding you cannot match and replace it all with new.