Len.
What size tyres do you want to make???
making a plaster mould should not be tooooo difficult, but moulding some from anything other than resin may be expensive.
Toy fairs always have loads of tyres for sale.
There are some rubber compounds out there, used for making automotive components, including motor mounts…but I have a feeling that this will be expensive for relatively small tyres….
A slice of suitably sized rubber tube did it for my simple landing craft …..
Ashley