Steve, did the tire just go flat due to a slow leak, or there a hole somewhere ?? If it is a radial tire and it has a hole, I don't think it supposed to be plugged.....there are special patches for those. My son-in-law patched 1 of his tires, and it ended up going flat again and disintegrated on I-40 in NC. before he found out......we learned a lesson from that.
Many tire stores have nitrogen available for fill.....but at a cost of course. If you decide to fill your tire with plain old air, it would be interesting to see if you experince any difference with that tire vs. the N2 filled ones, ie. higher running temperature, pressure increase, pressure losses, etc. It would be a good little experiment.
BTW, I am like the other posters, and run 80% N2 in my tires......plain old compressed air.
There are some that think the extra 15% (it is almost impossible to get 100%) or so of N2 is a benefit. Each member has their own ideas, and it has been discussed extensively here and other forums. Read the information, and make the decision of which way to go.