As you are an artisan who purchases wood to create wood products, your inventory would simply be wood in its raw material form (and price). According to the Canada Revenue Agency (CRA), Inventory is usually a list of goods held for sale or raw materials as well as packaging material and supplies, work-in-progress (goods and services that you have not yet completed at the end of your fiscal period), and finished goods that you have on hand.

For more information, check out: Canada Revenue Agency (CRA) - Inventory

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Last year I put Nil (as it was suggested to me by a turbo tax expert). Last year was my first year in business.

I don't have receipts for the raw wood materials. 

Do I need those as it would just be an estimate now.

Wonder if I should just put Nil again. 

If you don't have the receipts for the raw wood, it might not be a good idea to enter it in your inventory, unless you have some other way of proving how much it cost to the Canada Revenue Agency (CRA).