There are a couple of TOW ATGM shapes included in the one AFV Club Stryker ITV kit that has the TOW turret. Not sure if the Trumpeter kit has this included or not but having the AFV Club kit here in hand I was surprised to see the missile shapes included since these things are usually just a blur when seen in flight and good luck trying to model the guide wires trailing out of the back end of the missile.
If you are keen on modeling the SPIKE, I would suggest finding the dimensions for the missile and then look for something of suitable length and diameter to use as a stand in for the missile. This could end up as just a plastic rod shaped to resemble the SPIKE or maybe a couple of similar size in diameter missile shapes glued end to end to get the desired length. Considering the fact that the missile in question is not normally seen outside of the sealed launch tube you might be able to get away with using a MILAN, HOT, Dragon, or TOW launch tube with different end caps to get your desired results.