Go to Wal-Mart and try on some of their reading glasses to see what power works for you. You'll want to hold something out at the distance of the sights and then find what magnification makes the sights clear. You'll lose some clarity at distance, but that's the nature of the beast--you're going to have to trade something away once your eye can no longer accommodate enough to see things up close. And as long as the magnification level isn't too high, it probably won't be a big loss.
Write down the magnification power from the glasses that work best.
These folks sell safety glasses with various power magnifying lenses--match the power from the reading glasses that worked from your experiment and you should be good to go.
http://www.safetyglassestoday.com/ma...y-glasses.html
Another thing you can consider is getting prescription safety glasses. Because of the difference in the way glasses work vs. contacts, if your prescription is for nearsightedness, you can usually see things up close better with prescription glasses than with the same prescription in contacts once presbyopia sets in.