I am not sure if you are looking for a router/firewall or a AP/router/firewall.
I will tell you what I have set up and maybe it will help.
Firewall/Router: http://www.ebay.com/itm/HP-T5730-Thin-C ... 19dfe20909
I purchased the above (found one cheaper) and then purchased an expansion module kinda like this http://www.ebay.com/itm/DE961A-NEW-HP-C ... 2c71bdca81
And then an intel dual NIC card one port or WAN and one port for LAN (pfsense only works really well with intel chipsets) http://www.ebay.com/itm/2-Intel-Pro-100 ... 1c35c3eec1
I have pfsense installed on a 4gb flashdrive. I have a modem going to the WAN on my pfsense box. I have a switch going to my LAN port on the pfsense box. Then I have an AP Linksys WRT54GL with OpenWRT plugged into the switch.
So the pfsense box cost in total probably $75. I already had the wrt54gl but you can get those decently cheap too.
This may be totally overkill for what you are doing but it was fun to set up. A WRT54GL with OpenWRT is probably more than enough and is a full blown linux router.
So to answer your question no I don't think anyone sells open source kits but you can make your own for a decent price.