As stated Farnell do Lapp Kabel silicone cable in 3-core. It will be
red in colour, extremely flexible, fine stranded.

For a soldering iron you want silicone a) mainly so it does not tug on
the iron when in a stand or when in use and b) re contact temperature.
The downside of silicone is it will not slide over surfaces easily,
hence the better clothes irons with silicone cable also have it
covered in a woven fabric mesh. So a silicone iron cord might do,
although price "a bit samey".

Price is not bad, I'm afraid that is what it costs unless you can find
a generic "Performance Power" or similar brand which will be about 50%
less than the Lapp Kabel kind of price. If it were just a tough rubber
like H07RNF you can get that cheaply from www.10outof10.com -
basically a marquee or stage lighting supplier to shovel it by the
shed load. Someone on Ebay has 50m of H07RNF 1.5mm 3c for £38
currently which is rather good (might be asian manufacture which will
make seems to mean a stiffer cable compared to say Drakaflex, probably
rubber composition v ambient temperature).