where can i buy PET bottle fragment(from trash)

Dear KyleCai, 

There are several sources where you could get it, usually from
recycling elements.

In the United States, Blackport Market provides PET bottle fragments:
<http://www.blakeport.com/Pet%20Flakes.htm>. 

The Bottle Water Web <http://www.bottledwaterweb.com/> is a meeting
point between different enterpreneurs in the field of bottling. In
their message boards, I found the following, who sell waste PET
bottles:
<http://www.bottledwaterweb.com/board/index.cgi?read=3453> 
<http://www.bottledwaterweb.com/board/index.cgi?read=4085> 
<http://www.bottledwaterweb.com/board/index.cgi?read=4357> 
<http://www.bottledwaterweb.com/board/index.cgi?read=6106> 
<http://www.bottledwaterweb.com/board/index.cgi?read=7987> 
<http://www.bottledwaterweb.com/board/index.cgi?read=4391>
These are just recent examples, and the best thing would be to follow
the board and the quotes they offer.

Another good board to find business partners is Yahoo Groups' "Plastic
Scraps" message board <http://groups.yahoo.com/group/plastic_scraps/>.
For example, this message appeared there lately: -
<http://groups.yahoo.com/group/plastic_scraps/message/110>. 

Ali Baba is a similar board, where you could locate business partners
from around the world. In this case:
<http://www.alibaba.com/trade/offer/detail/11461208.html>. 

Ask 4 Plastic could be also a good source: 
<http://www.ask4plastic.com/index.htm> 

However, you might find it easier to contact a local recycling plant.
Since you did not indicate where you are located, I urge you to
contact your Ministry or Office of Environmental affairs, to find the
best source.

I hope this answered your question. My search terms were: 
"waste PET" "flakes OR fragments"
<://www.google.com/search?num=50&hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22waste+PET%22+%22flakes+OR+fragments%22>.
"waste PET" bottles <://www.google.com/search?num=50&hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22waste+PET%22+bottles>.
"waste PET"  <://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=%22waste+PET%22+>.
"PET bottles" recycling
<://www.google.com/search?q=%22PET+bottles%22+recycling&num=50&hl=en&lr=&ie=UTF-8&oe=UTF-8>
"PET bottles" fragments
<://www.google.com/search?num=50&hl=en&lr=&ie=UTF-8&oe=UTF-8&q=%22PET+bottles%22+fragments>
If you need any clarifications on this answer, please ask before you
rate/tip this answer.

For fun we used to pour liquid nitrogen into coke bottles, cap it
really tight, throw it into a field, and run like mad.  Boom!!!  Only
thing left of the coke bottle is little triangle-shaped shards of PET.

Greetings. 

If you are looking to buy huge quantity, I recommend you post your
offer to buy on http://www.RecycleinME.com. They will find you the
suppliers of pet fragments for free.