Where can i get a poke radar?

I think you get 1 in jubilife city

You need to see (not own) all 150 Pokemon in the Sinnoh dex. Then go to Sandgem Town and talk to Proffessor Rowan and get the national dex. After that, go to route 221 (south of Sandgem) and go to Pal Park which is at the very end of Route 221. Go in through the doors and you should see Proffessor Oak. He'll give you the PokeRadar.

You have to see all the 150 Pokemon in Sinnoh. Once you do that go to Sandgem town and talk to Prof. Rowan, then Prof. Oak comes in and he gives you the National Pokedex. He says that you should visit Pal Park. Pal Park is located at the end of Route 221. You'll need surf to get their sp bring along a Pokemon that knows surf. Once you arive to Pal Park, you'll see Prof. Oak again. He'll give you the pokeradar. The pokeradar will be in your Key items part of your bag. You have to be on short grass not tall grass in order for the pokeradar to work.

I think that when you get the national pokedex from prof. Rowan, that he also gives you a pokeradar, or Oak gives it to you when you go to Prof. Rowan's office.

First of all, you need to have beaten the Elite Four. Then, after you've seen all 150 Pokemon in the Sinnoh Pokedex, you need to obtain the National Pokedex from Oak in Rowan's lab in Sandgem Town. After a bit of chat with Rowan, he'll give you the Poke Radar.

You can only use the Poke Radar in Grass; short and tall. You can't use it on Roads, in Caves, or in Buildings.