Fantastic, you will love it! It is 20 years since I worked for Iwasaki over there so some places below may have changed, but I was last there very briefly about 5 years ago and there was still a great selection of lamp shops.
As others have mentioned, Akihabara Electric Town is probably your best bet to find interesting old lighting shops. Very easy to get to, just one stop on the Yamanote metro green line from Ueno central station in Tokyo.
Yodobashi Camera chainstores have an excellent selection of all modern lamps at perhaps the best prices. Probably you can still find some of the unique Japanese CFLs there, along with their unusual twin-tube circlines, the square circlines, flattened-section circlines, and flat spiral high wattage fluorescents which began to take over just before the LED era.
Yamagiwa lighting stores also had a good selection. Their main business is lighting fixtures, often high-end architectural and very expensive things, and to help customers keep old fixtures alive they used to be renowned for their excellent stocks of older incandescents, halogen and lower wattage HID types - although pretty expensive.
By far the most interesting for me though, were the small lamp and electronics markets. Akihabara is full of big chain stores that occupy most of the street frontage - but in between their huge and grossly over-lit entrances you can find narrow corridors, sometimes nothing more than just a staircase or escalator at street level, that extends backwards or upwards. It may feel like you are taking your life into your own hands to disappear into those entrances, but I can highly recommend exploring some of those! Those are the places that do not earn big profits and cannot afford big street entrances, and where you find the best deals as well as the oldest stocks. Sometimes it can be a bit hit and miss, it could be just a long corridor to a tiny smoky and room selling some kind of highly specialised obsolete electronic components. But other times it will open out into a cavernous space rammed full of tiny electronics stalls. The best ones to look out for are the entrances with undecipherable signs of text annotated with eg 2F, 3F, ... 7F etc - denoting the number of floors of markets that are stacked above the entrance, and here you can almost always find a few stands inside with some kind of older lighting products - as well as a mass of vintage electronics, which I also find fascinating to see.
There are also two fantastic small lamp shops I remember, but they were run by old Japanese gentelemen who even 25 years ago looked as though they should be retired so I am afraid that one day soon they may disappear. The first is
Koto Electric This guy still had carbon filament lamps and many older HID in stock last time I was there. See Google location and scroll through the photos at the link. IT seems the link does not work fully, you may have to click Back at upper left, and then click on the thumbnails again to see all.
Another good one was
Mitsuwa Denki but looking at these photos it seems they Ledified a lot (same thing again, click Back, then again choose Mitsuwa Denki from the list, and then Photos).
Akizuki has a lot of second hand old electronics, highly variable, I never found too much but worth trying. So popular it seems they often have a queue on the street to get inside! I was in touch with the owner by email a year or two ago, because he also gets stocks of older LEDs from the 70s-80s-90s, which I find quite interesting. But he did not have any stock left of the world's first white LED from Nichia of 1996 which I was looking for!
Marutsu I remember the name well but looking at these photos it seems they also modernised now and specialise in electronic components more. If I remember rightly they had 3 different shops in Akihabara so it could be that they have lighting at another location.
It would be worth you pinging a message to Al Martin, if he does not reply here. He was in Japan much more recently and found a great place that sold many older HID lamps, which I do not remember visiting myself. Perhaps he would have more details.
