The phrase "an initial supporter" is correct and usable in written English.
You can use it to refer to someone who was among the first to support a cause, project, or idea.
Example: "As an initial supporter of the initiative, she played a crucial role in its early development."
Alternatives: "a primary supporter" or "a founding supporter".
"You're talking about redeveloping a 50-block area," said a tourism executive who was an initial supporter of the stadium.
"I knew he was a nut on auto safety, an initial supporter of Ralph Nader's consumerism and an early listener to Rachel Carson's environmental theories.
Royal Dutch Shell, an initial supporter of climate legislation, said that it had told the institute that it would not participate in the rallies, although its employees would be free to attend if they wanted to.
Despite Iceland's status as a member of the pro-Western North Atlantic Treaty Organization, and as an initial supporter of the war in Iraq, it has found itself appealing to Eastern countries like Russia and, reportedly, China in a desperate bid for cash.
An initial supporter of the conflict, Jones led the effort to rename french fries as "freedom fries" in Capitol Hill after France refused to join the U.S. led coalition in the conflict.
President Obama was a strong initial supporter of the concept that the government should make sure Internet service providers don't act as gatekeepers of legal content.
And he was the initial supporter of Khan Academy, and CoderDojo.
Barack Obama's presidency, while far from being a failure, has been something of a disappointment to many of his initial supporters.
Slightly less than 2 percent of Obama's initial supporters have switched over to support Romney (the other 2 percent have joined the ranks of the undecided); and nearly 3 percent of Romney voters have reciprocated and switched to Obama (with another 3 percent becoming unsure).
Even initial supporters, such as Diane Ravitch, an education historian and former assistant secretary of education in George Bush senior's administration, realized how detrimental these measures were.
They were replaced with a narrow, base-driven strategy that alienated many of his initial supporters and undermined Rove's ambitious goal of creating a permanent Republican majority.
