Simple VPN device

Hello, I am looking to buy a VPN appliance to put between our router and
our firewall, preferably from Cisco as that is the brand of the router
we are trying to connect to. Currently this is our situation. We are
trying to establish a tunnel between our Firebox 700 with a sprint
managed router to no avail. We are thinking that we should purchase
something small from Cisco that would go between the firebox and our
router that would better be able to connect. Only a handful of PCs on
our network need to communicate through this tunnel so we don't need
something as robust as a Cisco VPN concentrator, but I understand that
they may make some smaller SoHo type VPN appliances that may do the
trick. Does anyone have any recommendations to help resolve this issue?

James