how to get entei

Just go to the global trade place and look for one

Or, you can use your Action replay, run in the wild and see a lv23 Entei.

Here is my best advice... You may buy/borrow a Pokemon Firered/Leafgreen (For GBA)game and choose bulbasaur as a starter. You will then play through the game until you have access to the Sevii Islands (which you will have in the later parts of the game.) The seventh island is where you will find the Unknown ruins. This is the tricky part... You will then attempt to catch every single Unknown (including ? And !), which is rather tedious work (trust me, I've done so myself, and it takes time.) You can use www.serebii.net to help you find their locations. Once you have obtained all of the Unknowns, then one of the three legendary dog Pokemon will appear in the wild. These options (Suicune, Raiku, and Entei will depend on the strength/weakness of your starter pokemon. So... Suicune for Charmander, Raiku for Squirtle, or Entei for Bulbasaur. Hence, is why if you want Entei, you choose Bulbasaur.) They will be tricky to catch, since they run away at every encounter and change routes when you do. It might be a lot of work, but if you succed, then you will have an authentic Entei. The second choice is to cheat, which I do not recommend for someone who puts their heart and soul into their game, or else it might... Malfunction. The third choice is to obtain it from the GTS... Although I imagine it'd be difficult (given that people ask for rare pokemon) and the high possibility that it would be hacked. These are the options that I could think of and provide to you, but it is your choice on what method you wish to take. Sincerely,
Alberio

You can put on the Pokemon Modifier cheat, type in ENTEI's National Pokedex number, run through grass while holding L are, and you will see Entei.

You can buy/borrow a Pokemon FireRed version for GBA. Pick BULBASAUR as your first POKEMON. Get access to the Sevii Islands, catch all UNOWN in the Unknown Ruins, and then run and ENTEI will appear.

You can obtain ENTEI through the GTS.

You can obtain ENTEI through trades.

You can obtain ENTEI through trading forums.