There is a gotcha to keep in mind. My trailer came with four aluminum wheels which are pretty thick at the bolt holes. The spare is steel and thus much thinner. The nuts one orders must have enoungh internal thread space for when the steel wheel is used or the spare will really wobbhle since the bolts would not tighten.
I got some just like Robert's picture from Advance Auto up the street. Be careful though, there are two types they carry, the difference is the back end - one is tapered and the other one is not. Take one with you.