Hi... you can try going to the Chanel boutique to get an original. Here are two ways I would suggest:
1) Just simply ask. Bring in your bag, tell them you don't know where you dust cover is (which is technically a true statement), and tell them you want to BUY one. They will most likely offer you one for free. SAs will not give out "free" dust covers, but if you bring in your bag and offer to pay for one, I would expect a nice SA will get you one.
2) Take your bag in for a deep clean/conditioning, and when you pick it up, ask them if you can buy another dust cover because you do not know where yours is.
Chanel does not give away dust covers for free. But if you spend money there (like cleaning a bag) and certainly when buying a new bag, and ask to "buy" a dust cover, they will most likely give it to you. Best way to get a new dust cover is when you are spending money with them.
Both examples above are from my own experience. Example #1 was at Bal Harbour where they just gave me a replacement cover. But... I do buy bags there. However, example #2 was in NYC where I was getting a bag cleaned. I did not bag the bag there, and just asked to buy another dust cover.
The downside is you may not get the "matching" size for your bag. They will just grab one from their back stock. But it will be Chanel.
Good luck and congrats on your new vintage bag!