You can pick up a cheap, plastic vernier caliper for a few bucks and measure the shank of the blade you're using now. (Assuming, of course, that you like your knife and want to keep using it.)
I like Barry King blades and I think they offer very good value for the money. As Buzzardbait says, in addition to the blades that fit BK knives, BK blades are available with a smaller 7/32" shank that fits Tandy knives. Leather Wranglers also offer excellent blades in different shank sizes. Can't speak about other suppliers due to lack of first hand knowledge. I lament the fact that swivel knife/blade vendors don't list the shank sizes of their products. It would make things a lot easier.
And remember that no matter which blade you buy, the vital thing is to keep it sharp so it performs at full potential.
Regards,
Arturo