You can find USB-to-PS/2 adapters on Amazon, NewEgg, or similar stores (possibly even locally, but they're not a very common item). For example, if you trust brand names, Belkin has the
Belkin USB to PS/2 adapter. If that's too much money, there's a less well-known brand
Perixx PERIPRO-401, PS/2 to USB Adapter. All I've seen look similar to those two, so you can probably search and find some others, as well.
Sometimes, your computer, keyboard, or mouse (or some combination of them) simply will not work with the adapter. If you read the reviews, you'll probably see this--occasionally, person will say it doesn't work. If that ends up being the case for you, perhaps you could try a different keyboard or mouse, or a different adapter if all else fails. I've used mine on a couple different MacBooks without any problem (I got one from NewEgg in 2005 or so and don't remember which one it was anymore--and I don't use it anymore). I wouldn't completely rule out the possibility that it simply won't work, but hopefully it works for you.
As a final note, there are actually two kinds of these adapters: "passive" and "active." I'm sure both of the two above are passive. They are likely compatible with any recent (i.e., produced in the era of USB, even if the keyboard itself has only PS/2) keyboard but may not work with older ones--or at least that seems to be the trend. Here's an an adapter that appears to be "active":
Ziotek PS2 Keyboard To USB Adapter. That just means that it contains a little chip inside to help with the conversion. They tend to be more widely compatible, but also more expensive. Note also that this one works only for a keyboard, so you'll have to buy a different adapter if you have a PS/2 mouse. Finally, you can see that this adapter is somewhat bulky, so depending on where you're putting it, you may want a small USB "extension" cable.
Good luck!