I can tell you for certain that they are available at the Philly PA and Elizabeth NJ IKEAs. There is a little display of all available counters by the same for the cabinetry, plus all the full kitchen displays also have counters, some of which are granite, etc.

Most likely each IKEA works with a local fabricator and supplier, so the supply will vary by store and that's why it's not on the website.

They sell granite and quartz, but it's like it is with all big box stores--those are just farmed out to a third party installer. The store is only the broker.