uxie,hippotas,hippodon and snorlax(yes he is in platinum).
There isn't a TM, but you should go to the move tutor in Pastoria.
Pokemon cannot learn Yawn in Pokemon Silver.
Yawn is not a TM.
There is no TM yawn. Only certain Pokemon can learn yawn/hypnosis/sleep powder/etc.
Slakoth
Yawn is not taught by a move tutor it can be learned by some Pokemon such as Snorlax also Slakoth and Slaking.
yawn
Yawn
At any gme stop.
There is no rental Pokemon in in Pokemon platinum
Untill the Pokemon wakes up.
Slakoth - Petalburgwoods Gulpin - Route 110