A pair of 1N5400 diodes will do the job nicely. Fog light bulbs pull just under 2 amps a piece, 1N5400s are rated at 3A apiece. 10p each from Maplin, order code QL81C. If they're not in stock, any 1N540x diode will do, they just have higher maximum voltage ratings.
Alternatively you can use a single P600A (or P600B, D, G, J, K, M) which is big enough to do both bulbs. 49p from Maplin, order code UK59P.
Remember, only one side needs to be done. Both bulbs will light because they're wired together already.