You can make your own silicone moulds- this Pinterest link to a
You tube video shows you how.
I made some about 6 months ago, using this recipe, and they are still flexible now.
Couple of points: It is indeed a very messy thing to mix, I wore disposable gloves.
Second, she says she left hers to cure(harden) overnight. I'm guessing humidity comes into this, as mine started to harden after less than an hour, so don't make up too big a batch until you know how long you have to work with it. But its great stuff, and works out way cheaper than silicone putty you can buy.
One more point- make sure you get Silicone Sealant, NOT Acrylic sealant. I picked up the wrong one first time round, and it sets solid.