Substitute for sesame oil?
Since your recipe only calls for a tablespoon, I am inferring this is toasted sesame oil, which is used for its strong and lovely flavor.

There really are no good substitutes for this purpose.

I would recommend investing in the bottle, which kept in the refrigerator should last a long time, and will bring flavor to many dishes.  

If you choose not to do that, I would simply omit the sesame oil, although your dish will lack certain depth and complexity the oil brings.