I saw a terrine recipe that looks interesting, but it calls for aspic. I'm not sure I can get hold of the bits and pieces I would need for that. Could I just dissolve a few envelopes of gelatin and use that instead?
Aspic is a type of stock which is high in gelatin, and which sets into a jelly when cooled. Unflavored gelatin will have basically the same mechanical properties as aspic, as long as the gelatin concentration is roughly the same (1/2 tbsp of dry gelatin will set about a cup of liquid). But the taste will be very different, of course: Aspic tastes like whatever it's made from, and unflavored gelatin will taste like nothing. So if you plan to use gelatin, it would be best to use stock instead of water when making it.
There are many recipes for terrines that use gelatin to bind them. Maybe look at a few of them so that you can zero in on the right amount for the quantity of other ingredients that you are using. I've seen 2 tsp to 4 3/4 tsp. This refers to powdered gelatin. If you are using sheet or leaf gelatin you will want to identify the strength (bloom), and be sure you are using an equivalent strength and amount.