Question about mirin: I can't find mirin but can find sake. Is there a significant difference between the two?

There is a certain kind of earthy sweetness in mirin, that sake + sugar combination does not have. I am almost tempted to say, although it will sound like a terrible blasphemy, that you are better off adding a touch of sweet vermouth to sake. Or maybe a tiny drop of mild honey.
Sake and mirin work so well together, because mirin provides that subtle flavor and distinct sweetness, while dry sake cuts into it and balances it out. I feel that you might achieve that effect better by adding a touch of vermouth, than sweetening sake with sugar.

Thank you, everyone, for all of the advice. I'll continue to look for mirin, but until then, I'll add sugar to sake until I find some.

Purists will say yes, but I would only say that's true if you are very familiar with the flavor of a particular recipe and want to be very consistent. Sake will work in a pinch. It is very difficult to find good mirin in the USA, even in Japanese specialty shops. The most common one is carried at Whole Foods Markets from Eden, but usually it is upwards of $8 for an 11 fl. oz bottle. My preferred brand is the Mikawa mirin from Mitoku, which you can find from various specialty purveyors online. If you find that you are cooking Japanese cuisine often, I would invest in a bottle of the real stuff. Ni Japanese Deli in NYC carries a full line of authentic Japanese ingredients - are incredibly helpful, and would probably ship non-perishable items within the USA if you contact them via phone or e-mail.

https://squareup.com/market/ni-japanese-deli
https://www.facebook.com/pages/Ni-Japanese-Deli/299297103454491?fref=ts

Mirin is like corn syrup. Nothing at all like sake.

Thanks for the quick response. I hope I'll find mirin when I look again for it, but I'm not optimistic, as I've been looking very intentionally for years. So if I want to sweeten sake to approximate mirin, how much sugar should I use? I normally only use a couple of tablespoons, but how much sugar should I use per cup of sake?

Here's a good description of mirin.
http://en.m.wikipedia.org/wiki/Mirin

Mirin is a little sweeter. I use the unseasoned version. The seasoned version is salty. I'm always just as happy using sake.

Most supermarkets have mirin in the Asian section. It's often called "Aji Mirin". It gives a bit of glaze sweetness to stir fries---you can sub a bit of sugar mixed with sake.