Reddit mentions of Black Diamond Oval Carabiner, Polished

You can make those biners work, they are by far not the best, but they will work. The optimum shape is oval.Something like this is far better for safety and line wear . A girth hitch is faster and uses less hardware. It also reduces the break strength by more than half . Using any knot will load the fibers unevenly and lead to premature wear and possible system failure. If you can find them, you can use a chain link with a 1-1.25 inch inner diameter. Just file down any burs and make sure it is loaded correctly each time. You could also make a shackle and pin lock if you have access. They are some of the absolute best home made locks.
There are many ways to slackline and it is super awesome that you are getting advice so early on. Like you, I didn't have any other slackers to learn from and made some pretty nasty mistakes. Broken bones, concussions, hundreds of dollars in gear destroyed. All because I didn't know what the hell I was doing. If I have any advice from all of my failure, it would be, Do it right the first time. Cutting corners and saving a few minutes can cost you a lot more in the long run than just doing it the safe way.
Here is a list of some good places to start learning if you are serious about slackstuff.

A good but in no way complete list of webbing and stats

Some line load physics

Destructive testings

What is shock load?

Some different pulley configurations

Winds effects on slacklines

A super easy, bombproof homemade line lock

The big daddy tension systems

A super handy anchor knot

If you have any more questions feel free to ask.