If I don't have Grand Marnier and don't want to buy a whole bottle for just 2 teaspoons, is there something I can use as a substitute? Can I simp...

Yup, i'd go for a mini bottle, available at most liquor stores. Or if you really don't feel like schlepping out, I'd up the orange zest to a full orange.

Get a mini bottle or sub orange blossom water.

I'd think the orange flavor would be pretty diminished if you left it out.

Do you have any other orange flavored liqueur around - e.g., Triple Sec or Cointreau? If not, a small bottle of Triple Sec only costs a few bucks - and you can use the rest in cocktails, like Margaritas. Grand Marnier is great to have around for desserts, etc. but is far more expensive.