Just buy a whisk, they're cheap and very useful.

Use a Granny Fork....or the biggest fork you have. It'll take a bit longer, but alternate arms and you'll get a good workout.

thanks! i thought of using a fork, but i wasn't sure if there were any better (i.e. less strenuous) options, but i think this is the best option!

Do you have an immersion blender? I would probably try using that. I wouldn't use a conventional blender, though, because too much of the icing would get onto the sides of the pitcher, making it difficult to incorporate the ingredients effectively. If you don't have an immersion blender, I think a table fork is probably your best bet. Make sure that the butter is nice and soft, i.e., room temperature. Using the same hand motion as if you were whisking it, beat the soft butter for about a minute. Rotate your wrist to make a good-sized circle with the end of the fork, to incorporate some air into the butter. Then gradually add the powdered sugar, continuing the same motion; sprinkle the lemon juice in after you've added about half the sugar. Continue to beat it hard with a fork until all the juice and all the sugar has been added. It won't be as fluffy as if you'd used a mixer, but it should be fairly light and easy to spread. Good luck!! ;o)

I am using powdered sugar, butter, and lemon juice.

What ingredients are in the recipe you plan to use?