depends on the lid your lifting etc, two main sizes of hole + anomalies. Ebay sell em, so do local industrial supply places. then you have options like long ones for easier lifting, or short for easier stowage. then there are the improvisations.
basically the possibilities are endless but the solution is simple: measure what you want to open, buy something that fits.
ha, looking at your introductory post and location i think many of can guess the system you are looking to enter. as i said, measure it! but as I'm feeling nice I'll help further: I think the smaller set of keys (as described by mike) fit the hole/ slot at the peek of the triangle, of if not you can at least use the thin end of the handle to some use on it, it has been known for people to use other DIY methods on those lids (including pliers!?! i used a DIY key). on that system some of them have the two sides of the triangle bolted together so the lid must be lifted out, it will only hinge when the sides are independent (dont know if you nead the larger keys for this, there not the heaviest of covers by any means). but just go and try!!!
as always make sure you dont leave the hole open and unattended, either barrier it or close it behind you -if it is hinged, but check you can open it from below first (with someone still on the surface), covers are a lot heavier from below.
make sure you follow other safety stuff, telling someone up top where you are going, how long you will be (longer than you expect, I'm sure time goes faster underground) etc etc. -theres plenty of stuff online you can start by reading.
Tested it on a small cover in my mates garden, and another one of those twin triangle affairs (do they actually have a name?) on the park near us. Opens dead easy!
Only worry is if like Bunk3r said, the two sides are bolted together...
Cheers for your help everyone, I'll post a link to the video of our explore when we've been