He's not going to have a particularly large pool to pick from--there are many that wouldn't even consider it. Half of those mentioned here have already pre-emptively turned it down.
"...A remarkable range of leading Republicans, including Gov. Nikki Haley of South Carolina and Senator Jeff Flake of Arizona, have been emphatic publicly or with their advisers and allies that they do not want to be considered as Mr. Trump’s running mate. The recoiling amounts to a rare rebuke for a front-runner: Politicians usually signal that they are not interested politely through back channels, or submit to the selection process, if only to burnish their national profiles...
"...Several Republican consultants said their clients were concerned that Mr. Trump’s unusually high unfavorable ratings with all voters and his unpopularity among women and Hispanics could doom him as a general election candidate and damage their own future political prospects if they were on his ticket..."
http://www.nytimes.com/2016/05/01/us...president.html