anyone know where i can buy a drill bit to cut 120mm holes. ive looked and found some but they are all for drilling stone

What are you drilling through? There are some bits you can use for plastic but not metal.

I got a set of 8 hole saws from eBay for a tenner, they not the best in the world but they work fine. eBay has loads of them

Personally I actually prefer to cut 120mm holes using a Dremel, it takes milleniums longer than a hole saw to cut a 120mm hole but hole saws can damage the metal you're working on if your not careful and let it over heat.

its for for cutting thin metal on a new project case i have. as you probably guessed it for my rad ventilation. case will be painted after cutting the metal also fan grill over it so the cut dosnt need to be perfect.
ill check ebay and see what i can find

If your not wanting to wait to get one from ebay etc you will be able to pick up one from a local electrical wholesaler. City Electrical Factos, Electrical Center, Kings, Newey & Eyre....theres a few. Tbh I am an electrician so i use that sorta stuff allot...and as your not a heavy handed fool you should be able to get through your case no problem. With it being new it will be nice and sharp so it should breeze through a few mm of pc case and scorching shouldn't be a problem....if you bit gets to hot just let it cool for a second...but i seriously doubt it....its not 10mm galvanized steel your going throug here!

excellent. thanks mate. will start a project thread when it gets underway.