We presently have one access point (LAN/WAN) on an individual subnet and two
hardwired subnets connected together using a bridge and a small Linksys
router with only a single LAN port. The two hard wired networks and one
wireless subnet are setup using fixed IP values. A second wireless access
point will be installed and turned into a free public access feed for the
coffee shop once we can properly isolate that subnet from the others.

What we need to wind up with is all 4 subnets being isolated from each other
while still being able to access the same internet feed. A few specific
devices may need to be routed from one subnet to another so we need to move
up to some kind of multiport LAN style router. A second WAN port with the
capability for load sharing would be nice but at this time the second WAN
port would not be needed.

I'm looking for suggestions on a router that will accomplish this while
being cost effective and available in the USA. At this point the charitable
organization I'm supporting does not care on what brand is used as long as
they are able to isolate the subnets while still being able to share a file
server one all but one wireless subnet.

The organization has specified that they do not want me to build a router
based on a PC, even though that might prove to be the cheapest alternative,
so please don't suggest that option.

I've tried Googling but I was swamped with the results. Too many of the
machines that turned up did not support actual individual LAN ports but
rather used a bridge. Hopefully people here will have had some personal
experience with a good machine that will do what we need.