rcbs.com, look at their products, moulds, click on the mould you are going to buy, listed right under the bullet picture there is the top punch number.
If your mould isn't listed there, try here:
http://www.castpics.net/subsite/CurMolds/RCBSMolds.jpg
p.s. If the bullet you are looking at has a flat nose, see if you can buy a "blank" top punch in the correct diameter. A flat nose punch will work and can be made at home. I used a 1/4" diameter hitch pin from a hardware store for a while. It has to be modified to work, but is easily done with a hacksaw and bench grinder.