To answer your original question - mine tin. Keep running through and mining the respawns. Every once in a while they will respawn as silver instead of tin. If you see a number of silver nodes in one area, chances are somebody was mining tin that wasn't high enough to mine silver yet, and when they respawned as silver he just had to pass them by from then on.
To make a few other points that people seem to be hung up on with mining and possibly save us from seeing this question again and again -
1) you don't need to mine any silver to raise your mining to the point where you can mine iron. Although smelting tin and bronze turn grey long before you can smelt iron, if you scroll over the tin nodes, you will notice that they are still green for mining. Skill ups from mining green nodes actually come surprisingly often, compared to what I seem to see from other professions - or maybe it's just that I mined more because the prices for the products were so good anyway. Either way, just smelt your bronze until it turns grey, then keep mining tin. If you happen to find the occassional silver spawn (which you will, if you are mining tin) - you just get a guaranteed skillup instead of the chance you get mining tin.
2) there doesn't seem to be a good place to get consistent silver spawns. The closest I have come is mining the tin in the lowest area that it appears in - apparently these are usually farmed by people with lower mining skills and the chances of them skipping the silver node because they are not high enough skill to mine it are somewhat higher. There isn't any point in trying to look at mapwow or allakhazam for all the places where silver spawns and then running around complaining that you're not finding any silver - the map of silver spawns is the same as the map of tin spawns, because occassionally a tin node will spawn as silver instead. Just mine every tin node you see - when you come back to it, it might be silver the next time, and in the meantime you'll probably end up skilling past silver to iron just from all the tin.
3) The same thing applies to gold (a rare spawn of iron) and truesilver (a rare spawn of mithril). There is no good tactic for farming the rare spawns - just farm the base metal and watch for the rare respawns.
4) very occasionally, higher level nodes will spawn lower level rare spawns - I have found silver where I had previously found iron or mithril (rather than tin), gold where I had found mithril and thorium (rather than iron) and truesilver where I had found thorium (rather than mithril). This seems to make areas of mixed node types particularly rich in rare spawn nodes - the badlands with mithril and iron seems to be fairly good for silver, gold, and truesilver due to mithril miners skipping all the silver nodes, and people who can only mine iron skipping all the gold and truesilver. Thousand Needles and the Charred Vale in Stonetalon mountains also seem to show this disparity to some extent. I don't think the difference is in the spawn rates of the rare spawns, but in the farming patterns and abilities of the miners present in those zones.