They can be purchased only at auto paint and collision repair supply stores. Evercoat Eurosoft comes in 20-ounce and half-gallon cans and includes the hardener catalyst; a 20-ounce can will last you a long time, but make sure it's well-sealed, as it will harden over time on its own in the can. Don't ask me how I know this. It's not the most economical; it's not cheap (about $20 for the 20-ounce can the last time I bought it) and there is invariably some waste, but for bulk filling it can't be beat. One nice thing about it is that it turns rubbery before it completely hardens; if you monitor it by gently poking it with the tip of your knife/scalpel blade as it sets, you'll know when. At that point, you can easily carve away any excess leaving a minimum to sand. And once sanded, it takes primer and putty just like styrene plastic because it's not really a putty but a "catalyzing styrene monomer," i.e., plastic!
I use old plastic coffee can lids or small ice cream carton lids to mix it. Once it's set, you can flex the lid and remove any waste and reuse the lid to mix the next batch.
There are other brands, but Evercoat seems to be more generally available.
Of the "spot" putties, I'd recommend either 3M Acryl-Blue Glazing Putty (retail $27 for a 21-ounce tube) or Spies Hecker Permacron Fine Putty 7715 (about $16 for an 8.8-ounce tube). These are lacquer-based, about the consistency of toothpaste, intended for filling pinholes and other small imperfections, and are good for filling seams. I apply them with a paint brush wetted with MEK (methyl ethyl ketone, the primary ingredient in lacquer thinner). Wet the brush, pick up a little putty right out of the mouth of the tube, and paint it on. They do shrink when applied in large amounts, but that's not what they're for. The 3M putty also comes in red and green, but the blue has a finer consistency and longer drying time, although it can be sanded in 30 minutes.
Interesting comment about Milliput. The times I have used it, I have found it to be softer than styrene.
Michael McMurtrey
IPMS-USA #1746
Carrollton, TX