One doesn't need to have THE wand to successfully perform magic
First of all we see in the books that it's not that a wand is either perfect match for a wizard or complete disaster. There is full range of intermediate stages when a wizard can use a wand with some success. For instance, in Deathly Hallows Harry was fine with Hermione's wand (though it didn't choose him), but was struggling with a snatcher's wand that didn't suit him at all. After getting wands from Draco and Bella, he chooses Draco's (though he doesn't know that) because it "feels friendlier" in his hand - like, it's not perfect, but better then the other one. Ron uses Charlie's wand and noone is bothered that it didn't choose him (same case with Neville).
So it can be just Ollivander's perfectionism and professional pride that he tries to find the perfect-perfect match for each new buyer. In fact, lots of wands in his shop would do just fine for a customer in case there will not be THE wand for them. He will just need to advice on the best choice.
Ollivander's is not the only wand shop
As we learn in Deathly Hallows after Ollivander was abducted, children are able to buy wands from other wandmakers. So probably if there is no match for them at Ollivanders, customers can try their luck in other shops.
Custom wands are possible
We know two cases when Ollivander did custom wands - Pettigrew and Luna (Deathly Hallows again). So if Ollivander wants to keep the customer or his professional pride is affected, he can do a custom wand. We do not know if he practices it though, but it still can be an option.