I wouldn't attempt to substitute.
I've read somewhere that you can use regular lime leaves, but I've never seen those anywhere.
Even Googling for lime leaf turns up kaffir lime leaves.
They can be found easily enough online:
ImportFood.com .
They freeze well for months in just a zip-lock bag.
The flavor profile is best described as a bright floral aromatic.
It's similar in function, not taste, to a bay leaf.
Thai cuisine uses kaffir lime leaves much as we do bay leaves.
If you decide buying online isn't worth the hassle, then Kitchen Savvy suggests the following substitute: 1/2 a small bay leaf 1/4 tsp of lime zest
1/8 tsp of fresh lemon thyme
cooking.stackexchange.com/users/60/hobodave
I agree that there is no true substitute, but if I were going to try, I'd use the zest of 1 lime for every 2 kaffir lime leaves.
I wouldn't do the bay leaf or lemon thyme suggested above.
cooking.stackexchange.com/users/1393/Michael Natkin
They used to sell them all the time at the Buford Hwy Farmer's Market (Buford Hwy & I-285), but have gone home empty handed that last few times I've looked.
Last time I bought them was sometime this spring.
The Atlanta Farmer's Market used to have them, but they've closed.
99 Ranch Market is closed.
Java did not have them.
Rumor has it that Your Debalb Farmer's Market might have them.
cooking.stackexchange.com/users/None/
There's the option of using Kaffir Lime essential oil - it is the best substitute I know of, much better than the dried Kaffir Lime leaves we can get around here (Israel).
It is truly wonderful.
Here's the one I use: http://thaifoodessentials.com/buy/
It's quite cheap and lasts for a long time (you only need a few drops per dish).
The website also has some instructions on how to use it, but generally, you want to add it towards the end of cooking, close to serving.
cooking.stackexchange.com/users/16920/Daniel Zoran
Just an FYI, I have been to Asian stores that used to carry Kaffir lime leafs.
They no longer stock them due to FDA (USA) import rules or so the owner told me.
He suggested calamansi limes.
cooking.stackexchange.com/users/None/
I used to buy curry and kaffir lime leaves at the DeKalb farmer's mkt (Atlanta), but have been told recently that the USDA has banned import in attempt to preempt certain microbes, bacteria, diseases.
Dunno what that is really about.
In the end, they are no longer available there.
However, if you live in a tropical, sub-tropical latitude, you can grow your own.
cooking.stackexchange.com/users/14165/Stephen F.
I bought mine from Amazon.com.
I'd found that there are different types of Asian grocery stores, and not all cater to all types of cooking.
For instance, Philipino stores don't necessarily carry what a Thai recipe may require.
So, even if you were to venture out, (Doug must not have driven in Atlanta)
you couldn't assume they'd have what you're looking for.
Go mail order.
cooking.stackexchange.com/users/6191/Susan 
I found them in Wegmans, and I've seen them in a couple of supermarkets - but just in a jar in oil.
Similar to a small jar of thai curry paste.
Same size, labeling, etc.
Not how I expected to find them, so be on the look-out.
cooking.stackexchange.com/users/1761/Alex
Kaffir Lime Leaves seem to be a common ingredient in Thai food, particularly coconut based dishes. I have  never  seen them in Atlanta, and I've looked (farmers market, whole foods, normal grocery, but not an Asian grocery store). What is the flavor profile of these leaves? Is there a good substitute?
What can I substitute for Kaffir Lime Leaves?
cooking.stackexchange.com
<substitutions><herbs><thai-cuisine><lime>
cooking.stackexchange.com/questions/8192
cooking.stackexchange.com/users/1259/yossarian
It might be best not to try to substitute, but you could try the zest of 1 lime for every 2 kaffir lime leaves, calamansi limes or Kaffir Lime essential oil   instead of dried Kaffir lime leaves. Try lime zest or kaffir lime essential oil.
There is no real substitute but you can try lime zest or calamansi limes. Kaffir lime essential oil is a good substitute.