As mentioned, no one can do it all, but you can be their 'One-Stop-Sign-Shop'.
Advertise as being capable of getting all of their sign needs met. Explain to them you don't do it all in-house per se, but have the ability to save them money in the long run.
Don't lie to them and tell them it won't cost them more going through you because any sensible person knows you have to turn a profit and that means higher cost to them... than you are paying.
That's why some of these discussions here on s101 and on the internet where people brag about doubling, tripling and making all kinds of profit to make sure they cover their costs in case something goes wrong hurts all of us. End-users can see certain information and get it in their heads that this kind of practice takes place in all shops and then become afraid to have you outsource for them. They feel they are being taken and just go on-line to find the absolute cheapest price.
We advertise as doing 95% of all of our work in-house, therefore assuring them the best competitive price they'll find for the work received.
You take a guy that's a broker and outsources everything.... if he doubled his/her costs on everything, he'd be making more than the guy manufacturing it for 1/2 the price. Wouldn't you want to go to the source... or at least try ?? That's why they have to be careful and only go 20% to 40% and hope that in the long run, they don't have any mishaps. The better notes and care they take to have everything hunky-dory, the better for all around.
Explain to them how it works and see how it turns out. Keep in mind, if you don't know anything about the new product, you'll be at the mercy of the wholesaler you choose, so make sure you're getting a reliable wholesaler. I believe your first line of defense to that would be contacting the wholesalers from s101 before going all around the world looking for price. That's what you're client can do, so look for legitimate companies to align with and keep your reputation clean and only use good people. Getting the cheapest one yourself will only backfire..... believe me. It would be better to turn it away then to have a disaster on your hands which will cost you in the pocketbook as well as your name on the street.