speaking as a WAHM diaper-maker...
1ml is slightly softer/stretchier than 2ml. A lot of diaper makers feel that 1ml is more comfortable for the child and they prefer to use it. 2ml will hold up to a little more abuse, and is typically used for wetbags or swim diapers (which get 'abused' by the chlorine in pools).
I also kinda like 2ml for larger size diapers because toddlers are going to fit the same diaper for longer, so it will be worn for, say, a year, rather than for just a few months. That means a lot more times through the washing machine too, so I'd rather have it be a little sturdier.
Realistically, unless you had samples of each side by side, you would probably not feel the difference. They are very very similar.
There are three types of PUL that I know of--fabrite is the original company taht does it. They have a variety of solid colors. Then there is somebody who makes an off-brand (also solid colors, but not quite the same shades as fabrite). I have tried the 'non-fabrite PUL' and it seems equal quality to me, but tends to be a couple of $$ cheaper per yard. They don't have as many colors, but they still have a decent selection
The last kind of PUL is going to include every print/stripe you see--everything except the original solid colors. It's "DIY PUL" which basically means that someone bought fabric yardage, sent it in to the company, and the company applied the laminate to the back of the fabric. My experiences with DIY PUL is that it *always* wears out faster than standard PUL. I was excited about it originally, seeing all these cute prints and things, but I refuse to use it or sell it now. I just find it really inferior in quality. A lot of people disagree with me on that, but I have made several dozen diapers for my own son and watched teh wear&tear develop, and the regular solid PUL holds up for SO much longer than the prints. So just something to consider.
As for places to buy it, there are two main options--if you want several yards (I'd say over 5-10yds) and don' tmind waiting a little while, then you can save quite a bit of money by ordering via a co-op. They buy full rolls straight from the manufacturer and are able to get discounted rates. Most WAHMs go that route. However, if you just want a few yards for personal use, I would just go retail...there are dozens of online stores out there that have PUL and other diaper making fabrics. I have bought from celticcloths and they do have good 2ml, but since I've mostly switched to 1ml now I've been shopping elsewhere. I like diapershop.com too, and there are quite a few others... I actually spent quite a bit of time compiling a list of links (as well as notes about what each store carries) and I posted it to my blog a few months ago. It's a marketing blog for my store, so I'm not allowed to just post the link on MDC threads, but if you PM me I'd be happy to give you the link if you'd like it.