The title of this thread relates to the use of 到 as a complement.
In Chinese grammar, the word ‘complement’ (補語) is mostly used as the shortened form for verbal complement (動詞補語 or 動補). As the name has suggested, a verbal complement is used only in conjunction with the preceding verb, and is thus connected neither to the subject nor to the object.
To be specific, there are 4 main types of verbal complements: resultative (結果補語), directional (趨向補語), potential (可能補語), and extent (程度補語). In Chinese grammar, 到 can be used only as a resultative complement. In the examples of 去到, 找都, 收到 etc, the first characters are the head verbs, indicating the process of an action; the second character 到 is the resultative complement, indicating the result of an action. The two characters together are called a resultative verb (結果動詞).
In Cantonese grammar, 到 can function as an extent complement (as well as a resultative complement), e.g. 行到好癐, 笑到轆地 ‘laugh oneself silly’. But 到 can’t be used this way in Chinese grammar.
As regards the phrase 买不到 as supplied by the OP, 买 is the head verb, 到 is the resultative complement, and 不 is the potential complement (indicating the inability of completing an action).