I'm having a machine shop rebuild a extra pair of heads. I have the castings, I picked up a set of springs and retains, and will get some valves too.

But where can I get the shims, stem seals, and valve guides ?

Does subaru sell a head overhaul kit?

I checked the gasket kit and none of the stuff looks like it's in there.

do I need to buy each piece seperatly?

If you dont have the parts.
The stem seals get them from Subaru, sold individually. Make sure you request intake and exhaust as they are different materials.
Shims, again from Subaru.

Valve guides, any head service workshop will be able to supply those. A common enough part.

You will find the stem seals are reasonably priced. Dont know about valve shims but the one piece buckets get a bit pricey, especially when you need to buy a whole set.

dan... I just checked the price of all the missing stuff from subarparts.com and the total would come out to $600 or so for everything I'm missing. Then I need a overhaul kit and then pay the machine shop...

It looks like the complete set of heads would come out to $1100, which might be a better way to go.

Any suggestions?

That would be an awesome price for my STI heads, make sure you are getting more than just the casting for that price.