Yes you can buy rucksacks online you try quencha it's available in low cost and quality is quite good , I have been using this wild craft karakoram 60 and trailblazer 55 pro so far I have had a good experience with it and a lot of stuff can be carried in it.
There are multiple ecommerce websites to order rucksacks from. You can select from various brands depending upon the budget. I would suggest to go for 60L rucksack as 60L would be enough for long trips also. my recommendation would be wildcraft flip ruck 2 rucksack 60L as it has a great quality and it comes with rain cover which is actually quite useful. I have used this rucksack on treks of more than 15km and the rucksack was comfortable enough without any pain in shoulder or back. You can check out decathlon rucksacks also if you have a price range of more than INR2000