Make sure of the size you have. If the bike no longer has its tires, then check on the rim. The size is often stamped on it opposite the valve hole. Some bikes imported here in decades past did come with 700c's.

If you don't really know too much about the bike, make sure it doesn't have tubular rims before you buy anything. If it does, rejoice. The tires cost a bit more but the ride is much better. The 27" ones were actually 700c. You'll mostly find those on higher end bikes unless it's really old.

If there is some side to side play in the wheel, if it seems like it can shake on the axle even just the tiniest amount, you need to readjust your bearings. The cones may already be shot. They can't take a lot of riding when misadjusted. If there were oily dusty stains around the axles then the oil could be creeping out of the grease. Grease is oil plus thickener and if the bike stood, the oil can creep out and leave just thickener which will be smashed out of the way and leave the bearing unlubed.

I like the Nashbar Primas in 27. They are cheap and have flat protection. They seem to roll easier than some other low cost tires.

The Walmart tire and the Performance Forte's are built more ruggedly. I got some 1" wide Fortes a while back. I didn't like the way those rode as much as the primas.

I had a set of Conti Ultra Sports in 700c. They rode great but flatted. They are available in 27.