I suggest you call Delta. It is discontinued product. You will have hard time getting that "O" ring from Home Depot. Chances are they will send you all you need for free.

Here is their contact info:

Delta Help Line at 1‑800‑345‑DELTA (3358). The Delta Help Line is available between 7 a.m. and 9 p.m. (CST), Monday through Saturday.

Email: [email protected]

In the United States:
Delta Faucet Company
55 E. 111th Street
Indianapolis, IN 46280

Let me know how you did. Good luck. Milo