Option 1: Universalis URL

At the end of every url for every item on universalis there is a number, for example using Orange Juice we can see it id is 4745 because orange juice has the url: https://universalis.app/market/4745

Option 2: item name list

go here hit control + f and then search this json file for the item you want, there will be the item id next to the name https://raw.githubusercontent.com/ffxiv-teamcraft/ffxiv-teamcraft/master/apps/client/src/assets/data/items.json

Option 3: build an id_to_item.json file with python

All item names are found here: https://raw.githubusercontent.com/ffxiv-teamcraft/ffxiv-teamcraft/staging/libs/data/src/lib/json/items.json

All marketable items are found here: https://universalis.app/api/marketable

To get replace the ids_to_item file with new ones do:

#!/usr/bin/python3
import requests, json

raw_items_names = requests.get("https://raw.githubusercontent.com/ffxiv-teamcraft/ffxiv-teamcraft/staging/libs/data/src/lib/json/items.json").json()
item_ids = requests.get('https://universalis.app/api/marketable').json()

marketable_names = {}

for id in item_ids:
  marketable_names[str(id)] = raw_items_names[str(id)]["en"]


with open('id_to_item.json','w') as writeData:    
  json.dump(marketable_names, writeData, indent=2)