need help with phones

hello i really like having the $45 monthly service with straight talk but i would really like a better phone the ones they offer on the site are not that good theres the GS3 but i think that runs off of sprint towers and the only decent service in my area is verizon i beleieve my area code is 56156 if someone could help me i dont know if i need to buy a phone off ebay or somethin and then buy a sim card and go that rout or what? if someone could give me some advise i would realy apreciate it thx.