Making purchases of second-hand musical instruments for myself and claiming them as depreciation/expense

Hello, I purchased a second-hand (so no invoice available) musical instrument before I started as a self-employed person. I still use it for my performances. Current market value (second-hand I would like to declare this as an expense/depreciation. What is the best way to do this? Purchases second-hand from myself (is that allowed?) and then declare it for depreciation? Or is there other way? Thank you in advance for your advice.