Specific mods like the Unlock All Catalogue Files V2 on ModdingWay enable every single item for free, including:
The most efficient way to unlock nearly all cosmetic items (boots, celebrations, kits) without needing to level up is by modifying the game's initialization files.
Go to the "Customise" menu and edit low-value players on your team to have 99 ratings. When you start a Career Mode (choosing "Current Customised" squads), these players will have massive transfer values (often 40M+), which you can sell immediately to boost your budget.