TL; DR - Não há nada de especial na varinha de Hagrid. Ele teria comprado na loja de Ollivander, assim como qualquer bruxa ou bruxo faria.
1) Sr. Garrick Ollivander em Wand Length: Da escrita pessoal de JK Rowling (canon) :
Most wands will be in the range of between nine and fourteen inches. While I have sold extremely short wands (eight inches and under) and very long wands (over fifteen inches), these are exceptionally rare. In the latter case, a physical peculiarity demanded the excessive wand length.
Assim, uma varinha especial pode ser feita se necessário para um mago de proporções anormais .
2) A descrição da varinha de Hagrid pela HP Wiki (não canon) :
The wand of Rubeus Hagrid is 16" long, and made of oak wood; the core material is unknown. This wand is the longest ever recorded, except for Lucius Malfoy's wand, which measured 18" in length. The wand is described as "rather bendy". This wand was snapped when Hagrid was expelled from Hogwarts School of Witchcraft and Wizardry in his third year, and either repaired or replaced at some point prior to his picking up Harry at his eleventh birthday. The wand was presumably concealed inside an umbrella.
Aqui vemos que a varinha de Hagrid era mais longa que o normal, mas não excessiva. T aqui não há razão para acreditar que Ollivander não teria a varinha em estoque.
Nós vemos aqui que JK Rowling está afirmando que uma das varinhas mais longas tinha 15 "mas que Hagrid tinha 16" e Lucius "tinha 18" ... o HP Wiki não é 100% preciso não é uma fonte para canon neste Mesmo assim, a citação do cânon implica que o Ollivander fará uma varinha personalizada, se necessário.
3) Varinhas costumavam ser feitas sob medida a partir da escrita de JK Rowling no Sr. Olivaras (canon) :
Prior to Mr Ollivander's proprietorship of the family business, wizards used a wide variety of wand cores. A customer would often present the wandmaker with a magical substance to which they were attached, or had inherited, or by which their family swore (hinted at by the core of Fleur Delacour's wand). Mr Ollivander, however, was a purist who insisted that the best wands would never be produced merely by encasing the whiskers of a favourite Kneazle (or the stalk of a Dittany plant that once saved a wizard's father from poisoning, or the mane of a kelpie a witch had once met on holiday in Scotland) in the customer's favourite wood. The best wands, he believed, had cores of immensely powerful magical substances, which were expertly enclosed in specially selected and complementary wandwoods, the result to be matched to an owner with whom the wand itself felt the most affinity. While there was initially substantial resistance to this revolutionary way of crafting wands, it swiftly became clear that Ollivander wands were infinitely superior to anything that had come before. His methods of locating wand woods and core substances marrying them together and matching them to ideal owners are all jealously guarded secrets that were coveted by rival wandmakers.
Aqui vemos que os assistentes podem trazer seus próprios materiais para que uma varinha seja feita se tudo mais falhar.