Well, it sounds like the OP has a stem with removable face plate (as he mentions two "halves") so he probably could put a 26.0 bar in his 25.4 stem. Don't do it, though, OP. Get yourself a 26.0 stem if you like your bar and your general well being.

Yes, almost all manufacturers make stems with various clamp diameters.

The most common are 25.4 (MTB, old cheap road bars and a few drop bars made for MTBs with drops), 26 old road standard, 31.8 newer MTB and road standard.

26.4 is ye olde Cinelli standard and there is at least one 35mm bar/stem out there.