Now I'm interested in these caps because (a) I actually like the TrackPoint pointing device, and anyway it's the only pointing device my laptop has at the moment, and (b) the default caps, while rather lovely when new, only take a few months to wear smooth and go black. The other two designs look like they may be more resistant to this, and anyway, (c) they look like they would be interesting to try.
If you live in the US, you can buy them online direct from IBM (for $9.99 plus tax and shipping, which isn't cheap considering what they are, but won't break the bank either). If you live in the UK, you get sent to a page which lists the contact details of a company in Massachusetts, of all places, but will offer to find a dealer. Unfortunately, this merely lists dealers which carry any IBM product; I haven't checked all of them, but Dabs certainly do not offer TrackPoint caps.
IBM UK has a Service Parts shop, at which you can buy all manner of stuff, including 2-packs of the ordinary TrackPoint caps. In fact I've already bought some from there in the past, together with a power lead. Should I want to, I can also buy a keyboard bezel for about a tenner so that I no longer have to have a crack in the side of my laptop. However, the aforementioned 6-pack of caps is a retail part, not a service part, so this shop won't sell it to me. Boo!