HondaCivicForum.com - thermostat housing

91 civic 1.5
hey all, i feel like an idiot. went to replace thermostat today, pulled out the two bolts the haynes told me to, and then when it wouldn't come off easily, the haynes said to tap it with a hammer, which i did. apparently there was a third bolt, and in hitting it with a hammer the bolt i hadn't seen pulled off a piece of the housing. so, my question is, is the thermostat housing part of the engine block, or is it easily replaceable? it seems to be bolted on with two bolts, but advance auto parts doesn't have it in the computer, and it's hard to tell because other engines i've dealt with have had the housing as a part of the engine block. any help would be greatly appreciated.
-scott