The screws for normal case fans that have head and thread pattern that you can't miss. You can buy them online and in bulk if needed while you can visit any local pc shop and they will sell you some screws. Unless you live in the Arctic or the Sahara then you are out of luck.
Hopefully this is a really simple one to answer. I've just purchased two 120mm Antec Tricool fans to add to my Antec 900 case but they didn't come with any screws or grommits.
What size screws (mm) should fit? I'm thinking 12mm self tapping, would that be right? Will ones from a general hardware store do the job?
Unsure on what the actual call-out are for those computer fan self-tapping screws, but if you can't find them in a local hardware store, you can use machine screws. Look for a 6-32, length will vary with the thickness of your fans. Typically for 120mm fans, thicknesses are 1", so give 1/4" for the nut, so go with a 6-32X1 1/4" screw, and don't forget to get the corresponding nuts or washers if you need them. If you're worried about length, you can always get them longer, and then dremel/cut them down to the lengths you desire.