Here are a few of the threads on THIS site that give links for finding the items:
http://www.neoitems.net/neohelp/showflat.php/Cat/0/Number/399833/an/0/page/0#399833

http://www.neoitems.net/neohelp/showflat.php/Cat/0/Number/403169/an/0/page/1#403169

http://www.neoitems.net/neohelp/showflat.php/Cat/0/Number/402275/an/0/page/1#402275

http://www.neoitems.net/neohelp/showflat.php/Cat/0/Number/399311/an/0/page/2#399311

http://www.neoitems.net/neohelp/showflat.php/Cat/0/Number/332489/an/0/page/2#332489

http://www.neoitems.net/neohelp/showflat.php/Cat/0/Number/396281/an/0/page/3#396281

There are many, many more if you use the search function.

As a genuine pack rat, that was an easy avatar for me. I was moving things between accounts one day and it just happened -- so I sent Solidus_Snake the info.
Start with the freebies and almost freebies:
Omelettes (whole, 2/3 and 1/3) - about 70 of them are cheap
Jellies (whole and half) - 14
Tombola items - around 30 are cheap
Fishing items - most are cheap
Advent - most Advent items never cost more than a few hundred. Check the lists by year under "Hints, Tips" on this site and pick up a few hundred more items.

Then move onto categories that have lots of cheap items:
Grooming - almost all of the old items are cheap. If you just buy the Lipsticks, Eyeshadows and Nail Varnishes you'll add 21 items, but check on the mirrors, shampoos, etc.
Low rarity items - On the database, enter a rarity like "10". There are 28 of those.
Island Merchandise - only a few items from the Tiki Tack shop cost more than a few NP.
Collectable Cards - the Blue ones are cheap
Desert Food - the burnt, scorched and ruined ones are only a few NP
Gifts - most are cheap
Gross Foods tend to be less expensive than others
School - prices plummeted when Neopets finally admitted that there would be no Neoschools. Check notebooks, pens and pencils.
Smoothies - another good source
Finally, a lot of the Usuki Doll sets and many Toys are cheap.