Best thing to do is probably just to buy another pot and transfer the glue over as you run out. You can theoretically use most non nylon brushes but it tends to dissolve the glue that keeps the bristles in place, so maybe rinse right away? I’ve used blunt syringe tips before with some success just using capillary action.
Do like these sculpts a lot. Great paint job. Would work well in all sorts of games.