It appears that Lampco - that used to stock SLI tubes, have now discontinued the sale of all discharge lamps.It certainly looks that way, Phosco152 with very few of the 'tabs' actually doing anything useful, apart from those for LED, fluorescent and halogens.
Interestingly, if you type in 'discharge' on their shop search, the full range still seems to be there, including 200W SLI lamps at £63.77 each (I think they are still cheaper on eBay though). Nb. I have not tried to add anything into my basket, so couldn't vouch if this 'search' is still operational.
That said, they do seem to have a separate website 'easy-lightbulbs.com' which seems to stock the majority of lamps including SOX. Perhaps this is replacing the Lampco pages.
I was intrigued to read that they were encouraging people to contact them, if they had any old /surplus bulbs that they wanted rid of.
They offer to collect them and will even pay you, should Lampco manage to sell these lamps on. it makes you wonder what they class as 'old light bulbs'?