Records and other physical media
CDJapan
My personal favourite for buying new Takanaka releases directly from Japan. Extremely reliable and quick. Easy to use website. Also highly regarded by many fans.
Guides and recommendations for purchasing records, merchandise and tickets. Made up of websites I personally use as well as fan recommendations.
Tickets
Buying Domestic Tour Tickets
Buying tickets to Takanaka's shows in Japan can be a little confusing. But it's actually not too difficult.
Head over to the link below. Scroll down to the "Reservation here" button. On the next page, translate it to your native language. Scroll down and click on the "Click here for overseas purchases (for those purchasing from outside Japan)" button. Now select the show you would like to attend. On the next page, select the quantity of tickets you would like and complete your purchase (please note that you cannot pick your seat location). Then, once in Japan, you will need to print your tickets at a 7-Eleven store with a code you are given after completing your purchase.
Fan Club
Joining the Official Fan Club
Takanaka's official fan club is on an app called "FANICON". Whilst the fan club is aimed primarily at domestic fans, it's also useful for overseas fans. You get to see occasional photos and content that's not shared anywhere else. And you also get presale access for his tours, including world tours. This is included with both silver and gold memberships. To join, check out the guide below that has step-by-step instructions. Then join the fan club.