N42 seems to be the wargaming standard and is what I've generally used on models. One small N35 might be sufficient to hold plastic troops in place on a movement tray (depending on size), but the price and availability (no harder to find than N45,
much easier to find cheaply than N50/52) of N42 makes sticking with them a slightly safer bet.
That said, a single 1x3mm N35 disk, if fit perfectly flush with the base bottom, is enough to keep smaller plastic infantry from shifting even slightly when placed on galvanized steel sheet, inverted, and shook with moderate vigor. Unless you're dealing with particularly heavy (large and/or metal) models, anything bigger is likely overkill.
Trawl Ebay. While the ones linked earlier are a bit small for their grade, you can definitely find sellers (primarily Chinese importers) listing lots of 100 or 200 in an appropriate size/strength for similar prices, often with free shipping. I've been using
these for magnetizing my Mantic Elves - 100 magnets of sufficient strength for the task, bought and shipped for $5. You're unlikely to get the same number of N42 in a similar size
quite that cheaply, but there are better deals out there than in your initial links, at least.
