Roller bearings in the steering post. I think its 4 in total, they are the same size as the clutchbell bearings - 5x8x2.5 iirc. Made a nice big difference to mine.

Check its not just the hex in the wheel that has worn/enlarged before you buy anything. Clamp on hexes can also help, depending on where the slop is, its hard to tell sometimes. Check the rod ends too, oh, and the pillow balls. Hubs, bearings etc.

I know how you feel, mines starting to get a bit sloppy too, most is down to play in the hub bearings though.