Hi Goriak,
The easiest way to obtain a Dragon Scale would be to simply buy one off another player through Trade chat, or to purchase them with PvP Coins, as it only takes five wins.
The second method would be to obtain them from wild Pokemon. Dragon scales do have a chance of being held by certain wild Pokemon species, however, this only applies to certain maps. I regret to inform you that Horsea in Vermilion City do not hold Dragon Scales. If you use the Reborn Bot in the official PRO Discord, you can use the following command: ^item dragon scale With this command, you will be given a list of all the locations wild Horsea and Seadra may possibly hold the item at. The closest spawns in which Horsea can possibly hold a Dragon Scale would be Fuchsia City, Route 17, Route 19, and Route 26.
I must add, by using a Pokemon with the move Covet or Thief, you are able to 'steal' a wild Pokemon's item without having to capture them. This will save you a lot of balls in the process. I would highly recommend using a Pokemon with the ability Frisk, and teaching it either Covet or Thief. (Some common Pokemon that accomplish this are Sentret and Wigglytuff). With the ability Frisk, your Pokemon will be able to see what type of item the enemy Pokemon is holding, which allows you to detect if the opposing Pokemon is holding a Dragon Scales. If the enemy Pokemon does hold an item, a battle message will appear telling you the name of the item.
As for your question regarding the daily quest, you can look at the Celadon Daily Quest Guide for more information.
Let me know if you have anymore questions, and have a great day!
Kind Regards, Astraea.