"You will definitely find products in different shapes and colors, exactly as you would find at shops and malls (which/where) the streets are full of them." In your sentence, you are trying to express two ideas: 1. The streets are full of shops and malls. 2. You will certainly find all sorts of products in the shops and malls. Idea 2 elaborates further on the the shops and malls mentioned in idea 1 (idea 1 mentions shops and malls, and idea 2 elaborates on these shops and malls' variety of products). For this purpose, we can use a relative clause such as a "where" clause to (i) express the idea of location (ii) provide more information on the nouns "shops" and "malls". Examples "The streets are full of shops and malls where you will definitely find [all sorts of] products." Alternative: "You will definitely find all sorts of products [in] shops and malls which the streets are full of." Here, you use "which" to specify or elaborate on something you are focusing on (shops and malls). Some writers regard the use of a preposition (of) at the end of a sentence to be ungrammatical while others find it acceptable. This is why I am presenting this second answer as an alternative. I hope this helps.

You have to use 'where' here; 'which' doesn't make sense.

Neither. The sentence doesn't make sense like that. The streets are full of what? Products? Then: "You'll find...… in shops and malls. The streets are full of them." (2 sentences) :)

No, the sentence doesn't work at all the way it is..

Keith - I see your point, but could it mean what I intend to, if I add a comma before "where", and maybe write " with them" instead of "of them"? I'm referring to the sentence that I have mentioned in the question.