This is a Coach Bolt, they are available anywhere. Also, you could probably get them off the shelf from any decent exhaust supplier, or an equivalent.

I would not waste the effort trying to save them, or the nuts. Cut 'em off and get new ones on: don't forget you will strain the other bolts trying to shift them, which means if you re-use them there's a good chance they will break in service later on. They are as like as not weakened by corrosion anyway, so is it really worth the risk for a few quid?

Don't be a cheapskate in this case - dosh out for a new set.

PS: I have just checked with my local KwikFit, and they change about 5-10 pounds for individual fitting kits.