Why does the Torah enjoin us regarding techelet? Because techelet resembles sapphire, and the Tablets were made of sapphire, to tell you that as long as Bnei Yisrael gaze upon this techelet they are reminded of what is inscribed on the Tablets and observe the commandments, therefore it is written, 'And you shall see it [the blue string] and remember all of the commandments of G-d and you shall do them.'
- Mishnat Rabbi Eliezer, Chap. 14
We are an authorized dealer of Ptil Tekhelet blue tzitzit and try to offer the best price available online, for both Rambam and Raavad. We are also set up to tie blue tzitzit on every tallit and tallit katan we sell.
Techelet is a complex issue, and I don’t claim to be an authority, but I can offer my customers some initial guidance.
In recent decades two vanguard organizations claim to have rediscovered the blue tzitzit dye referred to in Bamidbar 15:38. The first, Radzyn, is derived from the cuttlefish. The second, Ptil Tekhelet, is derived from Murex trunculus.
Today, most Orthodox who wear blue tzitzit insist on Ptil Tekhelet, although Chassidim (notably Breslov and Radzyn, of course) stick with the Radzyn tzitzits. Be warned that the Radzyn blue tzitzit dye runs somewhat, although they have apparently altered their methodology to diminish the problem. Your hands will have blue stains after tying the tzitzit, your tallit may get a bit smudged and the white tzitzits will get slightly discolored. From my experience the smudges come out with a damp cloth.
Whether you choose thick or thin blue tzitzit is a matter of aesthetics. The thick may be easier to tie. If you intend to order Ptil Tekhelet, first you’ll need to decide how you intend to tie the tzitzit, because the Rambam set enables you to tie the tzitzits so that one of the eight strings dangling from each corner is blue, while with the Raavad set you get two blue strings hanging on each corner.
The Ptil Tekhelet website has a wealth of information on the various blue tzitzit tying customs. Some of them are quite complicated, while others are fairly straightforward.