1. where can i find Cedar Spear?
2. where can i trade/exchange item that get from Plaza Point or Nyankoropon

Hi, welcome to forums.

You can get Cedar Spear from these dungeons by identifying the dropped weapons:

-Onigashima Training Grounds
-Kikaigahara
-Abou Forest
-Cave of Screams
-Daiku Cave
-Kokujou Forest

You can exchange your torn tickets for whole tickets in Onigashima. The NPC is black bakeneko cat called 'Yorozu Exchange' and is located by the giant iron mace.

You can exchange items for Onigiri Points in Onigashima. The NPC is a golden daruma called 'Points Daruma'. Points can also be bought, and then exchanged for gifts.

Alternatively, you can exchange certain items through the shopping plaza. Alt > Shopping Plaza > Nyankoropon > Point Exchange (green button). This is only if you absolutely have no use for the items and want to trade them away.

thanks
& here another question where can i find this 3 wepone
-Log Bow
-Sukedachi Sword
-Namari Taichi

Life Is Complicated

You can get those by identifying weapons from these dungeons:

Log bow:
-Den of Karma
-Kishinden
-Cave of Dreams
-Sakurajima Cave
-Oosumi Plains South
-Ooyama Residence
-Hoderi Shrine
-Hosuseri Shrine
-Hoori Shrine
-Three Deity Cave
-Iwanaga Cavern
-Oosumi Plains North
-Oosumi Plains West

Sukedachi Sword:
-Onigashima Training Grounds
-Kikaigahara
-Abou Forest
-Cave of Screams
-Daiku Cave
-Kokujou Forest

Namari Tachi:
-Ooikazuchi Cavern
-Oonaobi Shrine
-Yomotsuhirasaka
-Ankan Forest
-Well of Five Virtues
-Gedoumaru's Hideout

Sorry I forgot to link the wikia page previously. You should be able to get a lot of useful info there:
http://onigiri.wikia.com/wiki/Namari_Tachi