Where do I buy good weapons that is long blade?

Weapon Shops. Most towns have at least one merchant that sells weapons.
Vivec has many merchants with swords for sale.

What is your definition of "good"? How much money do you have to spend? Are you looking for an enchanted sword?

I'm looking for a weapon such a daedric or glass and yes.

You're not going to be finding powerful weapons like that for sale in shops.

Start exploring daedric ruins, or doing advanced quests for Guilds. Then you'll encounter some.
Also, you will start encountering glass and daedric weapons in Leveled Lists once you're high enough level. Fighting Golden Saints and Dremora Lords is a good way to find them.

And of course, a few can be found scattered through the world, but they are usually in very dangerous places.

well,if you can get to mournhold they have quite the selcection in their shops ,adamantuim weapons are at least cool looking imho.Thinks it only comes as twohanded claymore as adamantion longsword goes,shortsword is one hand,and it is what I usually get my self,and spear.Eventually the enchanter on second level of the craftsmen hall can have something for yoy as well.

usually I go over there early my self to trade a little spells,ingredients against paralyzis,and maybe a weapon or two,and then get out oof there to return later=the first trip directly to do my shopping and leave,next time is for real ,but then I feel I can survive in the sewers,can be hard down there

where do i buy Nordic long sword ?

You can get a Nordic Broadsword in Balmora at The Razor Hole.
A Nordic Claymore at the Orc Smith in Suran.
Nordic Silver Claymores and Silver Longswords are found throughout Solstheim.

You can purchase a Glass Longsword from Dronos Llervu in Ghostgate's Tower of Dusk.

I think your best chance at getting daedric and glass weapons is to kill Dremora Lords and Golden Saints.

The problem with these is that they only really appear in the wilderness and into the dungeons when your character is of a relatively high level already.

or find vassir dinadat mine and talk to dram bero and he will give u a daedric weapon at choice =)

You can also purchase Summon Dremora and Summon Golden Saint spells, use them to summon one or the other, and kill them. As the body falls, click on it to open its inventory and if it is wielding a Daedric weapon, it's yours for the taking. Golden Saints will also carry Daedric Shields and Daedric Tower Shields, as well as glass shields, in addition to Daedric and Glass weapons (amongst others). Killing summoned creatures is usually how I procure my weapons, but if you're not strong enough to fight them or have enough magicka to cast the spells to summon them, you're out of luck until you are ready.

You can purchase bound weapon spells if you need a quick and dirty magical weapon to kill them with, as normal/non-enchanted weapons will not harm Daedra.

If you are level 20+ the Golden Saints spawn a lot in the Grazelands. All of my characters go on Deadra hunts in the Grazelands when they have nothing better to do.

you can find glass stuff in ghost gate. daedric shrines are also baddass if you can survive the guards.

Most Nordic silver weaponry can also be purchased from the smith outside Thirsk.

Right!
But get your lvl2 char to Thirsk.

good very much depends on level, early on with my warrior character i found myself a demon or devil battle axe (forget which one now), which if you use its enchant becomes a bound battle axe - IE a Daedric Battle axe - for about a minute as i recall - you can buy demon/devil tantos etc in the small shop in Balmora though they do cost a few grand, however since the real thing costs about a hundred times as much it'll do for starters, next step up for me was umbra, had that sword for ages, go look him up "if you think you're hard enough"

It's not impossible if you're willing to do some sneaking. Staying close to the shoreline makes it possible to escape into the water if you can't handle the enemies. But yes, it can be a bit challenging.