Plenty of vendors of aftermarket parts for Land Rovers, for example Paddock Spares, John Craddock, Brookwell 4x4, Island 4x4, mm4x4, etc etc. I tend to use Paddocks or John Craddock for most items, saves dealing with loads of suppliers, but for costly items I'd search a bit wider for a keener price.
Beware - its a minefield out there! I tried to buy standard aftermarket shocks, and ended up with 2" lifted ones, maybe I was unlucky but in searching and deciding what to buy, I came across a LOT of misinformation.
After reading a lot of posts on here about the troubles with lifting, I decided to keep mine standard height. If you do lift it and its a 300TDi Disco, at a minimum, in addition to the shocks and springs, I'd convert the rear prop coupling from 3 bolt rubber doughnut to 4 bolt. There's been reports of the doughnut lasting only a very short time (like, a month) just after it was lifted, when previously they were trouble-free. I'm sure other people have had no issues, and others have also found additional parts which wear quicker or do odd things (eg props, caster correction on steering). A search should reveal lots of info on it.