Plain floorboards?

[JP_]

My current (and partially rotten) floorboards are plain, in that there are no tongue and groove.
All the ones I find online (Wickes, Travis P etc. are t&g). Can I buy plain ones anywhere? They're all going to be screwed to joists, so not really sure why t&g is so popular?

 

[foxhole]

Any timber yard, don’t ask/look for floorboards , just need suitable size timber.

 

[big-all]

possibly ex 6x1" planed planks [21x143mm]
wood shrinks and gaps = draughts
if you point load a plank it can only distribute the load along the plank
if its txg it spreads the load to a much greater area