If you want molds like flowers or stars and your formula compresses enough that you would be able to get it out, I would go for silicone. If you have a craft store like Michael's nearby, they have silicone molds like that. They also have regular sales with coupons; up to 50% off.
Some chocolate / candy molds also have great designs. They may be a bit small but then again, they can be used in multiples for a bath, or single for a foot soak.
If you have a dollar tree store in your area they have some cute shaped ice cube trays. They usually have flowers, sea shapes, stars and holiday shapes.