Pubs help, but there's a certain point where you get good enough so that pubs don't really help that much anymore, most pubbers have pretty awful movement so you kind of get used to how they move. I'd say HL lobbies can be very helpful too since hopefully people have marginally better movement in there, but I don't play very many lobbies. If you can try and find a decent pub to snipe on, like skial upward or skial harvest.
I've never played on a sniper server in my life so I couldn't tell you whether they are helpful or not, I'd imagine it's just a bunch of second scoping.
aldertr_newbots wouldn't help
tr_newbots has helped me a lot, though at the cost of a bit of sanity since trying to hit the scout going at mach 3 can be pretty hard. In terms of the best warmup these days I'd say either this and the one DM server you can snipe on helps, but I don't think I recommend the DM server in good conscience since I'm pretty sure every hates it when there's a sniper in DM, including my own team.