Where can I purchase UHD 4K Blu-ray blank discs? I have a 4K television, 4K Blu-ray player, and a 4K burner, but am unable to find blank discs.
You can’t find something that does not exist.
Recordable blanks have not been designed to match the specifications (physical and logical) of a commercial 4K pressed disc. For instance, there is no recordable equivalent of a double-layer 66GB or triple-layer 100GB UHD BD-ROM.
Some 4K Blu-ray disc players may accept properly mastered 4K discs which have been recorded on 2, 3, or 4-layer BD-R but that would occur on a case-by-case basis and there are no guarantees. As an example, the spec sheet for my Sony 4K player categorically states it is incompatible with high-capacity BD-R media.
You would have better luck feeding a video file (e.g. MP4) to that same player via a USB drive or BluRay data disc. Besides that - what mastering software were you intending on using?