Local recycling and revolve centres (often based at local council dump and rubbish tips)
Ask your local tradespeople if you could have the empty reels when finished with them. Plumbers, electricians and telephone contractors laying cables are a good place to start as are nearby building sites.
Electrical supply stores
Drive around the local industrial estate – often wooden reels are left on the kerb for pickup so they don’t need to dispose of them (although some do return the larger ones to wholesalers to get back their deposit apparently – but you might get lucky!). You can also get an idea of what businesses use them if you see them piled up in the yards and can go back and ask for the empty reels when they open again.
If you are brave enough you could stop at roadworks and ask if they have any you could pick up when available or head into your local council workshop and ask there. Make sure to explain to them what you want to use them for.
Online marketplaces like Gumtree often have them listed in the free or giveaway sections.