Amazon is your best bet, although difficult to find any under 30cm. You can always snip them to length. There's always little contact patches for soldering every 3 LEDs or so that you can cut them at
I had a look at the per metre ARGB strip at Pimoroni, they offer 3 densities of LED which is nice. The 3 pin connectors they offer wouldn't work with a PC ARGB header as they are 4 pin with a blank, so you'd have to make something up for the PC end. You could probably make 5 or 6 for about £20, soldering them up yourself. My new board has no LEDs or a power/HDD light, so would consider a 15cm strip myself