You can buy the Sega Genesis cartridge but be aware this game is now rare, out of print and expensive to buy.
There are other cheaper options to buying Phantasy Star 1
You can buy it on Gameboy Advance in the Phantasy Star Collection
Another option is to buy Phantasy Star 1 on the Nintendo Wii virtual console shop channel for 500 points.
Another option is to buy Sonic's Ultimate Genesis Collection for either PlayStation 3 or Xbox 360 disc but you have to unlock it first by meeting requirements.