Knowing the series and its fairly whimsical tendencies, Ollivander probably has it accounted for. Or maybe they could have an über-epic two-handed wand that they could spin around like a double lightsaber. Or they could use one with a straight handle, although it would be rather anticlimactic.

I had to look at the comments to really get it >.>

As for an ambidextrous wizard, I don't think which hand you hold the wand in matters. The wand chooses the wizard, and while you have to reverse a scissor to grip it comfortably in the hand opposite the one it's meant for, that doesn't mean you have to get a totally different one.

...Why did I just use scissors in a wand analogy?

I imagine he would be delighted by an unusual and therefore "tricky" customer. ^^ I'm not sure about wand lore in general, but with Harry Potter wands, they seem to be based more on personality of the witch or wizard rather than things like being right or left handed, or ambidextrous. So firemagic is probably right that the question is simply to help start with a smaller selection.

The Harry Potter wiki has some interesting information about wands. Wand Lore and more specifics on the "laws" of wands. This guy actually makes wands, so his lore is probably more general than specific to a fandom, he even has a book about it!

And this forum is for Harry Potter wands, it has a lot of interesting information about wand woods, cores, etc.
*grumbles at my wand woods and their characteristics*

On another note, and apologies if this has been answered, or come up in a mission, I haven't read many Harry Potter missions. I was thinking as I went to answer the question, "If I ever do a Harry Potter mission that ends up in Diagon Ally, I'm going to have my agent go get their own wand." And that left me wondering if that was allowable. So if one's agents were in a Harry Potter mission could they go purchase their own wands? I know several agents have Sue wands they've nicked from defeated Sue's but could an agent buy their own, or for that matter anything else out of wizarding shops, if they had some downtime in Diagon Ally? If so, would they have to be a witch or wizard agent to pull it off, or would it be enough that they were in disguise as a student, and therefore a witch or wizard in world? As to the lack of pay that cause most agents to just pick up free stuff from a Sue over buying it, they could either nick it from the Sue, or if they had a small reserve of their own, we know Gringotts changes money, so it would purely be a problem of whether the world recognized them as magical and therefore Ollivander wouldn't be thrown for a loop when muggles come looking for wands, or if it were possible, but only for witch or wizard agents.

First of all, nobody is truely ambidextrous. There is one hand that is ever so slightly dominant, but for ambidextrous folk, the difference is so slim they hardly notice it. So, perhaps, Ollivander knows a spell that determines the dominant hand.

Another answer is that it truly does not matter which hand you use, but he asks for the comfort of the user. If you could use both hands equally, and are comfortable using either hand, then just grab some wands and we shall find the one for you! Or something like that.

The third is that there are no ambi wizards, and you are doomed to go wandless forever, becoming the Harry Potter equivalent of Rincewind. This is the least likely option of them, but there you go.

Okay, I just got an image of Rambo Potter with dual tommy wands, and it is awesome.

But, all joking aside, I think he would just find a wand that could be used in both hands. I'm certain, being a wand crafter, that he could pull it off - it's never said in-verse that it isn't possible, or that it is possible, leaving the readers to infer what they will.

Well, in the first place 'the wand chooses the wizard'. It always seemed to me like the whole measuring thing was to narrow down the choices. So there are probably sets for right and left handed people, and sets for ambidextrous, and whatnot.

It's not like he just goes 'oh, wait, you use both hands? Get the f*ck out of my store.'

Or maybe he just goes with whichever one you favor? Even with ambidextrous people I'd expect them to have one hand that they use a bit more over the other.