I've lived in Tokyo for several years, but I still always head to Akihabara to buy gear on visits, without question!
kakaku.com shows DOS PARA has that 10th anniversary model in stock and ships in Tokyo within 24 hours. If I were you, I would talk to the staff at the hotel you are staying in and ask them to help you send it directly to the hotel. If they are helpful, they can take your cash, help you 振込 (furikomi) or direct-transfer the payment at an ATM or a convenience store for a few bucks.
Another alternative is to have the staff give the company a call at:
DOS PARA
TEL:0570-028-120 (PHS・IP電話からは03-4332-9194)
FAX:0467-79-8231
電話受付時間 (telephone hours)
平日:9:00-21:00 (weekdays)
土日祝:10:00-18:00 (weekends)
and find out if you can go down to the Akihabara store directly and buy/pickup the keyboard (use google maps to find the store). Be sure to bring a print-out of that kakaku.com page so they know exactly what model you want.
imho, kind-of-nutty to use a proxy when you are actually there, but then again, being able to speak some Japanese would help even though most hotels which cater to foreigners will be able to help you in English - gl