Anyone know where I can obtain "sieved coir fibre" as outlined in the project to make an alpine trough?
No, but you don't need it, that is an old recipe for hyper-tufa. Sieved peat would do, or even sieved non-peat potting compost. Anything which is fibrous is what the mixture needs. And the sieving is only to make sure there are no lumps of material in it.
Look up super tufa on youtube, it will tell you how to do it