The easiest way to start taking advantage of the power of Facebook LAAs, is to create a lookalike audience from people who viewed your videos either on Facebook or Instagram. It is obviously easier to collect 500 users who watched specific videos than to collect 500 users who purchased from you. Obviously, the quality of the audience will not be as valuable as a Purchaser LAA, but you can try experimenting with it and test if it brings you better results than a cold interest based targeting.