Easy and cheap antigen/antibody couple
A good choice is the IgG/anti-IgG pair for cheap, easy to demonstrate/work with system. E.g. Goat-IgG coupled with donkey anti-goat IgG.  Just about any antibody supplier will have these.
These type of systems are the basic primary/secondary antibody bindings routinely used in immuno-assays, blotting etc. These may be mouse IgG coupled with a labeled anti-mouse IgG for example.
Reference
Wilson MS, Nie W. Electrochemical multianalyte immunoassays using an array-based sensor. Anal Chem. 2006;78(8):2507-2513. doi:10.1021/ac0518452