1. Easiest method is using
You must be registered to see the links
you have to create account first though (use google translate, very easy). After account creation you can top-up Bitcash point using western CC (Visa/MC/Amex etc), or use Alipay+ (last/bottom option) to bridge the transaction to your local payment processor (not western CC). With Bitcash point loaded just go to DLsite and buy directly (use Bitcash as payment methods obviously).
2. Buy DLsite point from known authorized sellers, read this:
You must be registered to see the links
3. I usually bought from Dl-Pay (now dead) or offgamers (currently unavailable), but since the payment methods change last year I bought it from local point seller on my country (with some mark-up). It's a bit more expensive but much safer (local account, no Visa/MC/paypal).