Credit Card I am petrified to get one because I dont want them to run my credit?

Right now i am trying to buy a house and i need one more revolving account i have two my car note and target but i need an additional one to boost my credit score. I dont want them to run my credit because i cant afford another inquiry. my friend told me to just apply for a babies r us card because i have a baby and i can buy things that she needs but im scared of that inquiry that comes with th territory. I was thinking about a secured credit card through my credit union but the apr is 9.99% i think that thats a little high and the minimum is $250 plus a fee of $83 for whatever reason. so can somebody give me some good advice on what to do? My credit score is 519 you all know that i cant afford another bad hit! lol!