Whenever I found a PI that researched in what I am interested in (antibiotic resistance) I contact them asking if they were expecting openings in the near future. I found a PI that was hiring a post-doc, which I don't qualify for, and asked if he was expecting to hire a research assistant in the near future. He told me his lab is expanding in antibiotic resistance research, which is the only field I want to explore now since my current lab on type 1 diabetes made me realize I am more of a cell/microbiology type. Since I was interested in a very specific field, it took me months to find a PI that researched this field and happened to be hiring for a position I qualified for in the near future. However, given I was interested only in antibiotic resistance, impressing the PI was a simple job and he said he'd hire me when the lab expands in a couple months. This doesn't only benefit me. It also benefits the PI since they won't have to go through dozens of applications to hire someone who's a good fit.
If you do what everyone else does, wait for job postings then apply, it makes it hard to stand out.
Also, yes. It is hard to get into a lab. It took me 8 months before I found this PI. I contacted like 50 before him and all were something like "Sorry, I don't expect any openings in the near future." It just depends on you and how badly you want it.
Also, just so you know, a lab assistant is grunt work. Your work is not researched based. It's more like washing cages, preparing equipment. Very little research work. At least from my experience.