You can buy screw removers but I have never seen them in sizes for small screws like the ones, I am sure are in your lens mount. One thought though, is that you might try to ask a specialist supplier such as a jewellers' supply company if they can help - they might have or be able to suggest something.
A better option would probably be trying first is to see if you can recut the screw head /screw slot in situ. Again you could go to a jewellers supply company and buy a chisel graver of the sort used by this guy in the video (jump to about 6.00 minutes in). They are the ones used with a hammer not the sort designed to be pushed by hand and are sold singly, are not too expensive from memory and would work I am sure as they are made from hardened steel. Just recut a slot and use a normal driver that fits that slot to unscrew it. If necessary regrind an existing screw driver specifically to fit the new slot closely.
Another alternative if you have a Dremel tool is to use a small Dremel burr to recut the slot instead of a metal graver. There is a reference in the following thread to a guy doing exactly that for a small camera screw. Dremels are great - I love them and would say this solution is probably your best bet assuming you have a steady hand and good eyes. Also assuming you already have a Dremel or can borrow one. Unless you want to buy one of course. BTW if the existing slot is phillips head you can still turn this into a normal screw slot for the purpose of removal of the thing.
Also if the screw is tight (and it may be given the screw was stripped) you could try a rapid heating / cooling cycle to break the lock. Use the tip of a small soldering iron (the sort used for electronic PCBs) and hold it to the head of the screw to heat it. Then use a blast of cold air (a giotto rocket blower or canned air could work) to rapidly cool it again. Then try to unscrew it. On the chance that thread lock has been used you may also wish to first try using some acetone to make sure the screw will unlock once the head slot has been recut.