best to do it when the turbo is off and you can get in there, you don't really want to be pulling everthing back off in the near future to replace a blown gasket. also there might be more than the two that are gone or about to fall off. if you handy on the tools you can do it yourself, there is a write up somewhere on this site that shows the procedure for using a stud extractor, should be able to find it easy with a search.
i've just had to do a few of mine, three of which were missing and another 3 just broke off when i put the spanner over them, and after all my searching for thred pitch and diameter it seems that all the rb's are and M10 x 1.25 stud (both ends) about 50mm long. about $2 - $3 each.
to get the broken ones out i just use a left twist drill, works a treat, no stuffing around with the possibility of snapping a stud extractor or easy out. one thing to keep in mind is the limited space, you may need a right angle drill to get on the stud nice and square.