I've built three rifles so there are people here with far more expertise than myself. My first build I launched my front detent spring, with the detent of course far into the wild blue yonder. I'm still waiting to find them walking barefoot around the house years later. I would get thee to Lowe's or your hardware store of choice and get a 1/4 clevis pin. I did this for my next two builds and it paid for itself the first time I didn't launch a spring and front takedown detent all the way across the room. Arfcom has some instructions with good pictures on how to use it.
I honestly never found a huge amount of cost savings in building just a plain jane AR unless I was willing to wait and literally buy it piece by piece over the course of weeks or months. I never built an upper, however, so the calculus might be different there. My understanding is though that you have to get special tools and need a good vice on hand so if you're just doing a one off it's not really worth it unless you can borrow tools.doing since it's fun and easy to do, but for me the big advantage was to buy exactly what I wanted for the lower and not have to swap out parts. Plus it's fun to do.