No you can't. If you want to buy one then you can get them from the atheletes shop at the pokeathelon dome.
the gyms are in the citys but some are harder to get into than others
You need to get further that the 4th gym leader to do that
i find the easyest way is in every route keep walk around until you think you have caught all them in that area if you are want the to get the pokedex full just battle through out the main quest then go back to begining and go through the citys and towns for the missing Pokemon not the best answer but i hope you get what i mean :)
go to ever grande city and defeat Pokemon league. affter that go find these legendarys listed.rayquazalatiosregi'iceregirockregisteel.after you are through with that then go catch every Pokemon possible in Pokemon sapphire.after that trade with Pokemon ruby to get the Pokemon only in ruby. Then buy a gameshark for GBA. you need it to get Pokemon from difrent regions such as lugia ho oh celebi -etc. when you have every Pokemon from these regions,hoen,jhoto and kanto then go to lilycove citys game editor. he will give you the diploma. you have then beaten the game.
After you deafeat your 4th badge from Flannery, Brendan/May will say somthing about battling petalburg citys gym. Go to Petalburg Gym and go through the door until you reach Norman! If you have more Pokemon Questions my profile is Kyogre286.
Well, go to Goldenrod city, if you know where the flower shop is, talk to the women in there and you will have to have got the citys Gym Badge.
you surf to them
go to snowpoint and Canalave Citys
vertanturf and slateport
just about everywhere except citys
After you awaken Groudon
i think they have one in ethier the celadon citys mall thing, or the game corner
tropius.. pastoria citys great marsh =]
level up your Pokemon and get many full heals
the gyms are in the citys but some are harder to get into than others
I think it is a mewtwo in armour.. I thought that because of the tail
in the house beside enterna citys gym