Splitting shingles is definitely one of those tasks where the correct tool is recommended. You CAN bodge it with other splitting tools, but you will have a lot less control. The froe not only gives you a nice straight split, but it lets you control the direction of the split through the grain of the wood, depending on which way you apply pressure on the handle.
It is also worth noting that good shingles need so additional processing. Typically a drawknife is used to smooth and straighten the surface and edges, so that they sit nicely against each other, as well as to reduce the weight of the finished roof. Shingles also taper from a thick end to a thinner end.
Even for the fairly small number of shingles you are looking at making I would recommend getting one.