Easy and cheap antigen/antibody couple

For an application I need to find a cheap antigen and cheap correspondent antibody. The antigen can be literally any molecule that is cheap and potentially easy to produce and with a correspondent antibody that is cheap and easy to produce as well. I imagine this antigen will need to be a molecule that triggers very easily an immune response to result in a easy to produce antibody.

I have never worked in immunology so I don’t know what system antigen-antibody to choose. I know pregnancy test are common so probably the antibody for Human chorionic gonadotropin (hCG) is easy and cheap to produce but I don’t think hCG is.

Are bigger molecules better than smaller ones as antigens? Any suggestion is helpful.