You'll never find it at a parts store. You have most of it--maybe all of it--already.
Right now, you have a lever on the column, when you move it, it turns the column, it moves linkage down to the transmission. Also, in moving the column, it allows the key to be inserted and removed in "Park", and it moves the shift indicator on the dash.
You need to add the floor shifter to move the transmission lever...but you need to KEEP all the rest of the linkage, so that when the trans lever moves, all the rest of the linkage moves, your column moves and your shift indicator works.
You'll have to modify the column so that your original linkage works in reverse, (instead of the columm lever moving linkage down to the trans, the trans lever moves linkage up to the column) without having to lift the shift lever on the column to go through the gears. I've never done that. I imagine there's some sort of spring interlock, and you'll have to dick with it to make it all work without the lever on the column.
GM did all this on a bazillion Camaros and Firebirds, for example, with floor shift. Almost certainly on other floor-shift cars as well.
