You can create these items without knowing the appropriate spells, but for each spell prerequisite you don't meet, the Spellcraft DC increases by 5.
Rarity has nothing to do with it. You need the Craft Wondrous Item feat, some money, some time, and a skill check.
I dont really think they are rare, if you are thinking about shops that would sell it.
Like the above poster said , you dont need to know the spells you use to craft many items , which is the reason crafting wondrous items is so great.
You just need to be good at spellcraft.
Based on how many characters own one or both, I'd say they're among the most common magical items, after potion of cure light wounds and +1 longsword.
The Handy Haversack has a listed CL of 9.
So, for your lv. 3 caster, the DC to create it would be 5 (base) + 9 (CL) + 5 (since you don't know the spell) = DC 19.
So your caster needs to have Craft Wondrous Items, 1,000gp, 8 hours time (i.e. 1 day) and is able to beat a spellcraft check with a DC of at least 19.
Note: If you can reliably hit a spellcraft DC of 24 (in this specific state) to decrease the crafting time by half.