Almond powder can be obtained by grinding peeled (and preferably roasted) sweet almonds. If you lack a grinder or would try for a somewhat different texture, you could crush them in a mortar or run them through a food processor or equivalent.
The ice cream is likely to freeze quite solid - consider using some alcohol to soften it slightly, eat it fresh soon after churning, or just remember to take it out from the freezer about 20 minutes before serving.