Amazon has a pretty good deal. 24 flavors for $35.
Buy the lot and you pick the flavors.
If you have never used LorAnn oils, go to Michaels or other retail seller of candymaking supplies, and buy the DRAM size (1 teaspoon). Buy a few and try them. Some of the oils have colour that will come through in the icing.
Then you can order the ounce or 4-ounce size direct from LorAnn by calling their toll-free number (888-4LORANN) if you do not order online. Also ask for droppers (packs of 4) when you order the larger bottles--then replace the caps permanently to use these very concentrated oils without any waste.