You should have a search on websites like Amazon, Ebay, Etsy, Alibaba, Dhgate etc., These websites have a good collection of pen cases.
Dropshipmate shopify app helps you to import products from all the above sites for your wood cases and also many websites listed in this app from where you can import unlimited products into your shopify store. This app helps you to enable Daily Auto Update Sync also.
I suggest to add reviews on your products which helps you to drive traffic onto your website so that you can make sales as you are going to do dropshipping.
All the Best...