Most any model live steam seller will have the exact item. Especially the UK.
That said, teflon tape rolled into a string or cord and then wound around the glass and pushed down into the packing gland works well.
Agree with PTFE tape - thousands of different applications can work - if originally designed for graphited (cotton) packing. - I even ran an oscillating engine (worn) with ptfe tape wrapped around to replace the grahited string packing for piston rings. - Not a durable as the graphited string in dynamic applications, but showed me all the other problems with the old and worn engine.
Personally, I simply buy good quality silcon o-rings for all sight glasses - but ONLY TIGHTEN a tad more than the pressure required for sealing - Well over-tightening will cause the glass tube to crack or fracture under "seal pressure" which make not be detected until the boiler achieves a temperature and pressure to develop the crack to failure. (Differential expansion of the length of boiler versus length of tube between copper and glass when heated 150 degrees from cold! - You can't beat Physics!). I tighten gland nuts tight by fingers alone, then hydralic test the boiler to 1.6 times NWP and check glasses don't leak - or use the spanner a fraction of a turn until a leak is sealed. - That way I never have a problem running, and glasses have lasted decades in my boilers. - N.B. ALWAYS check and ensure fittings are truly aligned by using a drill or precision steel rod of the same OD as the glass tube to align fittings when tightening to the boiler. A mis-aligned fitting will cause stresses in the glass tube to be exacerbated and risk failure when in steam.
Do Not use liquid silcon seal as it often does not cure hard enough not to creep under pressure when in steam. - It depends on the manufactured grade of material, and few commercial grades form a hard enough silicon bead to resist boiler pressure.
By all means use it for the side of the bath though... I spent many years trying to cure leaking oil from motorcycles (1945~1970s variants) with "liquid gasket" silicon. But when I worked with a Japanese motor manufacturer and localised the Japanese liquid gaskets, the Europeans were amazed at the various formulations used by the Japanese, as they were beyond the applications and specifications required for most "general" European market products. To the extent, they declined to supply some materials and we had to import the Japanese products. But I learned "the West" had a lot of "low density" products, while Japan had a lot of "high density" applications and products. So maybe there are some liquid silicon products on the market suitable for gauge glasses, but not in my experience. - And O-rings cost "$1 for 5...".