Where's the best place to get them?
Best place is directly from Haze. Go the the Cans and get $5 off (the XL batteries are an add-on to a can order w/ a discount)
I got the XL's and couldn't be happier, however, I had zero problems with the regular batteries, which I had been using for several months. So a "good" set of regular batteries should work fine - they just have less juice, and after a few cans, they take awhile to warm up. I do think that the XL's have a faster heat-up time overall - but I haven't actually measured it.
Far as actual vaping, once the oven is at temp, it should vape well with either battery. I use the cans exclusively, at temp 3, and those herbs are well vaped and I am well baked