Heres a link for an online union supplier, the 3" seems like a good deal but the 4" er is WAY too much. I've never been able to find a 4" for less than $49.99 any where online though.
http://www.plumbingsupply.com/pvc.html#union
Oh, and a link to their Bulkhead fittings page too.
http://www.plumbingsupply.com/bulkhead.html