where do you get them? Online mostly (with a few exceptions). Probiotics and enzymes are from specific companies for example.
how do you know the quality of what you get? You have to research or trust someone who has. For example with probiotics I look at research on how much survives the gastro tract, does it colonize, does it survive transport temps, is there research showing this strain and brand work, etc. Omega 3's, if from fish, I want to know don't contain heavy metals and has third party testing to objectively prove that. Enzymes--the companies make different kinds and they do develop their products in certain ways. So what you select might be affected by what you're hoping to accomplish.
Is it a you get what you pay for proposition? Mostly it seems to be. I think this might because companies that are going to prove their product works, do research, test effectiveness, make improvements, etc. are spending money to do that.
Some things don't matter but then you don't see huge price differences generally either. Vitamin D for example you want a D3 form but I wouldn't worry about brand. I have found that certain brands (often online) sell in larger quantities though (like 5,000 IU) that make it more cost effective.
We use everything you mentioned (and more) so if you want suggestions based on my research and experience with different products I'm happy to share. We've also dealt with some pretty complex health issues and problems here and I'm happy to share ideas/what I've learned.