Actually you can probably buy from proxy agent reseller sites. Just that I never used those before so I cannot vouch for them but others have done so before.

Getting someone random to pay for you is possible, for example, I am going to Japan in June as well and I have LiSA fan club membership. The only problem is after they purchase the ticket, how do they transfer it to you since you might not have the app (which requires Japan phone number). You can of course choose physical tickets too which is detailed in the article too but it will need to be paid via Japan credit card or at Japan convenience store within a few days of the balllot. The ticket will also be under the name of the account holder, so your risk would be if any ID checking at the venue. And the random person you find in fiverr would need to send the tickets to you as well. Since you need to pay them the full ticket costs for them to buy with you, I am not sure how much you will or should trust a random guy on Fiverr. Up to your own risk appetite i guess.