How to contact the Republican Party
As in the whole party? Take out an ad on Fox News Channel. Actually unlike the democratic party who march in lock step the republican party is a bit like herding cats. As the more free of the two parties the weakness is that they are too free and given to wandering to 3rd party candidates and such. Democratic freedom of thought is punishable by shaming and having a barbell fall on your neck accidentally...
AS such contacting all of the members in the Republican party is hard/impossible to do. If you wanted to contact the lead of the Republican party you have a few choices. Donald Trump is the de facto leader of the party right now (scary I know). Reince Priebus however you spell his name is the RNC chair, his buddies call him Rinse Prius. Paul Ryan is the house majority leader, he has been known to windsurf. Mitch McConnell is the Senate majority lead, he is part turtle I believe. I jest because this stuff is so dry that in a light breeze it would blow away and never be seen again.
Good luck in your ventures. The individual contact information can be found on the internet.