I've always bought the games on steam but after seeing the limited/collectors edition for sale in japan, I gave in and started collecting them. All of them were from 2nd hand shops.
My favourite has to be the bracer edition for sky first chapter.
Just sharing my recent haul. Majority of them were from bookoffs and mandrake (2nd hand) and yodabashi (first hand). These stores also offer tax free compared to other stores.
Some gachapon places also had the flat gachapon machines that dispensed the character stands for 12 rider series. I thought these were pretty cool though the gachapon aspect + how uncommon they were made collecting all 12 a bit hard (and expensive)
Edit: typo in title, DIY not DYI