If you're after buying new, try your friendly local rigger, for second hand advertise here, ybw.com and any other yachting board you fancy.
If you look in the Selden catalogue, they give a way of calculating appropriate length and diameter for a whisker pole for any yacht. Length is likely to be J or a bit more, diameter will be dependent on the yacht's righting moment (which they tell you how to work out) and pole length. For the same pole length, the diameter for a whisker pole comes out less than for a spi pole.
Giving the size you want in an ad, rather than "for an S31" will save you from getting useless responses to your ad.