https://wiki.project1999.com/Hand_Made_Backpack
you should get ~2 of these.
they are for sale any time of the day and week in EC tunnel for 50pp a piece.
then depending on your class (STR and needs), probably Medicine Bags, maybe a Fletching Kit / Sewing Kit, etc.
As for dropped special bags in the cheap category:
https://wiki.project1999.com/Lionhide_Backpack (150pp or lvl 25 mob)
https://wiki.project1999.com/Archeologist_Pack (100pp or lvl ~25 mob)
https://wiki.project1999.com/Travelers_Pack (100pp or lvl 25 mob)
https://wiki.project1999.com/Shralok_Pack (125pp or lvl ~21 mob)
These are also often found in EC tunnel.