It depends where you live. If you live in the US, try Apoxie-Sculpt for resin and Deluxe Plastic Putty for styrene, you should be able to get them through Amazon. If you live in the UK, Sylmasta sell a good resin putty too.
You can sand the Apoxie-Sculpt and the Sylmasta resin putties when they have cured, and you can smooth them with a wet finger for about 10-20 minutes after application and before they have cured.
I've never been keen on Milliput or the Squadron fillers (if you can still but them), but have used the Apoxie, Deluxe, and Sylmasta putties.