There are a few different swarms in the D&D range, there are two very good swarms in the "Desert of Desolation" range -
http://www.wizards.com/default.asp?x=dnd/mg/doddemonweb swarm & rot scarab swarm.
Unfortunately these look more like giant red ants and large scarab beetles than spiders, but they are very useful pieces. Just over an inch in diameter with insects a couple of milimetres long.
The D&D sets have a limited run, but the Desert of Desolation should still be reasonably common on places like eBay.
Earlier sets might not be so easy to get hold of.
The "Unhallowed" range has a very good rat swarm that you might still find, but the spider swarm is quite an old one now. Very good, solid and sturdy, but perhaps hard to come by now.