I bought all my prefolds online. I got some from Green Mountain, some from Cotton Babies, and a couple that came with my Econobum covers. They’re all fine, I honestly don’t have much of a preference. The Green Mountain ones have colored thread on the end to help identify the size, and that was helpful; we started with the orange ones and moved to the yellow ones when DS was a couple months old.
I’m not actually sure how many prefolds we have. Around 3 months or so we started using Fuzzibunz OS instead of the prefolds and covers, so now the prefolds are just burp cloths, and we have them all over the house for that reason. 36 sounds like plenty to me, though if you have a bigger baby and plan to do prefolds for more than a month or two, you might find yourself wishing you had more than one size.
I feel like we must have been doing something wrong, because we almost never reused our covers without washing them. DS used to soak the prefolds so thoroughly that the covers just felt yucky too. We probably could have wiped them off and left them to dry for a couple hours, then reused, but we just bought lots of covers instead. I’ve heard of people getting by with just a few, but I think we had 2 Econobum, 3-4 Thirsties, 2 Bummis, and 2 Flip.
Re: weight, if you look at growth charts, it looks like the 50th percentile for 15 lbs happens around 4-5 months. Also, remember that even if it says “15 lbs” whether or not it fits will also depend on your kid’s body type.
Re: detergent, we use Country Save which we buy in bulk online. And we use a dry pail.
Also, remember that if your infant is exclusively breastfed, you don’t need to spray off the diapers before they go in the wash, since all the poop is water-soluble. We tried to spray off all the diapers in those early days when he was pooping 45 million times a day and it made me hate cloth diapering, until we figured out that we didn’t have to, and then life was so much nicer.