[Cattieclaws (OP)]

Hi guys. I fancy having a go at creating synthetic opals:
https ://link.springer.com/content/pdf/10.1007%2Fs11051-016-3691-8.pdf

But I’m wondering if anyone could advise on an affordable and more obtainable alternative to Tetraethyl orthosilicate?

Many thanks.

[chiralSPO]

Unfortunately I think you would have great difficulty finding a better starting point for making these.

The rate of decomposition of the silicon-containing starting material is crucial to the size (and size distribution) of the nanoparticles. So if you start with a different silicon source, you would have to redo all of the optimization work (and I'm guessing, since you're asking for a cheaper/more accessible alternative to tetraethyl orthosilicate, that you don't have an electron microscope or dynamic light scattering setup on hand to check on the size and distribution of the particles formed—you would just have a binary analysis: it worked or it didn't work—and it probably won't work first time...)

Tetraethyl orthosilicate (TEOS) is not particularly expensive (about $1 per mL), especially since you would presumably only need small quantities. I don't know where you are, or what sort of safety equipment you have available, but it shouldn't be impossible to get online...
Safety Data Sheet: https://www.sigmaaldrich.com/US/en/sds/aldrich/131903