Ghillies originally didn't wear what we would today recognise as a ghillie suit, they wore individual tartans tailored to each specific estate. It wasn't until 1916 that the Lovat Scouts (drawn from Highland estate workers, gamekeepers, and stalkers) wore what we would typically associate with the term - they were also the first sniper unit.
I'd actually avoid the typical "full" ghillie suits in favour of leaf suits - if the environment permits. Military suits are, by design, multi-environment suits with different demands - specifically range of engagement and leaf suits provide greater concealment at closer ranges, at the expense of flexibility. Check out companies such as Stalker, Jack Pyke, North Mountain Gear, and Arcturus. Failing that Ebay has a massive selection of cheap suits that act as perfectly adequate bases without spending much money, and you can occasionally come across Russian military leaf suits by Leshy and Voin.
Typical British Military M.O. is to use a ghillie base with neutral hessian/raffia and a host of foliage loops, then attach natural vegetation to suit - the usual term is "Cobra Hood" or "Viper Hood" or even "Recon Hood" but they all refer to a base garment that allows customisation (Such as those made by TCS) but there are many, many companies out there copying each other. I've used
Arktis in the past to great success,
Webtex and
Giena Tactics.
Another option is a Russian Sniper Suit such as
these from Grey Shop or those from ANA Tactical.
In short, there are many choices depending on playstyle.
Garnish is the real consideration and I was always told either 60:40 or even 70:30 Natural to Artificial. Natural should be fairly self explanatory but there are dozens of options for artificial garnish and whilst I wouldn't necessarily recommend buying from Skirmshop, they do have a
representative selection which includes the "new standard" Haloscreen and Nanoscreen.
I'm not going to get into pattern management and light reflection and absorption, this video covers it in greater detail: