Silver casting help needed

Hi,
I am new to a lot of this has so apologies if this is a daft question!
I need to send a mould off to a caster which I have never done before. One caster has advised I can send them my metal clay version that I have made but as I need a few different sizes this seems an expensive way of doing it. Another has advised I can send them a mould in clay, wax or anything that isn't too porous.
The mould I need is an acorn cup which I've been making in metal clay which is proving too expensive.
I only have basic kit - so I'm assuming my butane hand held torch won't heat silver enough to make a mould using delft clay?
Any suggestions as to the best way I can do this would be really welcome as I'm going round in circles with it now!
Thanks

3D printing seems an obvious option if you need different sizes.

You could cast your self using Delft Clay, and a hand held torch like this:

Thanks Dennis.