There are numerous sources online, but we've found the people at HADLEIGH ENTERPRISES LTD both knowledgeable and helpful. No kickbacks for us, but I believe good service should be rewarded.
They can cut tapes to virtually any width which can be very useful.
We've recommended SCAPA 3507 for frameless windows for a long time. This is GREY, but there is also a BLACK version, SCAPA 3509 too if you prefer. We recommend 3mm for frameless but other thicknesses are available. Both are single sided (ie adhesive on one side) with a foam core and a semi-skinned upper surface.
They also gave us a sample of SCAPA 3259 (BLACK) which appears very similar. Ask David about this as I'm sure he knows the difference.
As a slight aside, this looks identical to the drip seal spare which used to be sold by Lewmar (MPN 360051999) for sealing the opening part of the now obsolete (and rare) Ocean Port.
Contact details for Hadleigh below.
Contact David Ogles
Hadleigh Enterprises Ltd
Unit 11, Buckingham Square
Hurricane Way
Wickford, Essex
SS11 8YQ
+44 (0) 1268 572255