Does it matter what bolt i use for a valve cover as long as they match the size of the originals? I lost a couple and need to replace them but not sure what/where bolt to get.

 

In addition to size (length, diameter and thread pitch) and style of the head, bolts come in different grades (strengths) appropriate to the application. There are also variations in material and finish, etc.
Something like a valve cover requires several bolts. Take one of them either to a dealer to get identical replacements or a junkyard to find replacements as close as possible or to the hardware store to likewise get as close as possible to what you need.

You don't specify what it came from but I would assume if the car is from the not-too-distant past the bolts will be metric sizes. Old model cars may have SAE sizes.