DEFINITIVE pregnancy test?

Throwaway because reasons.

Gals, I need help. My boyfriend (24M) and I (21F) have been together for three years and have had an active sex life for about a year and a half. Even before we started having sex, I have had somewhat irregular periods: Skipping months, being late a few weeks, etc. My mother is the same way. He has been my only sexual partner and I have been his only sexual partner. We have had one other pregnancy scare in the past, but after one of those home pregnancy tests came back negative (and me starting my period the next day) we both agreed to not freak out if I'm late. We always use a condom when having sex and he always pulls out.

Well, I'm late again this month by about 4 weeks and I made the mistake of telling one of my close friends, who is now pregnant, about it... She says she even got two negative test results (one from a store bought test and one from a doctor) before she eventually found out she WAS pregnant. Last night, we took a store bought test and, again, it came back negative.

My question is this: What's the BEST WAY to find out of you are for sure pregnant or not? Neither of us have very good health insurance and are pretty naive about this whole situation and kind of freaking out about it. I mean, a negative is a negative right? Where is the best place to go to get a test done for cheap?

Sorry if my writing is a bit sloppy, its been a stressfull few weeks as we are NOT ready for children yet.

If you think this could go on another subreddit let me know, i just don't know where else to turn for information...

Thank you