Spent a while looking at this - struck out at the volvo penta store. No prices on complete transmissions & a lot of obsolete part numbers. If you want an o-ring, you're good - the whole thing? Forget it.
Your transmission is probably v-p #852731, which has been replaced by #872830, then by #873078. No google hits on any of these numbers, however I did find a hit on #873077 - the 2.37:1 version of that transmission. This was on eBay for a NEW transmission - $6650.
Hope this helps..