We have an 18yr old Marshall pocket coil mattress that we came to love.... a bit too firm at first but after break in was great. I am a 200 lb side sleeper and my wife is 130 lbs and a side sleeper. In the last few months one side only is causing me to awake from pressure points... mainly shoulders. I hate a hot mattress so have no interest in memory foam. I have been researching latex. Also wondering about a latex topper. We live in the Toronto area (hence the Marshall). Don't wish to drop $3000 on a mattress we might not like. Contemplating another Marshall or a latex at a place we can switch out of if not happy. Any advice would be great.

You are ahead of the game.  You know you like the feel of Marshall coils.  After eighteen years you can be certain that you got your money's worth out of your mattress! 
Check out new mattresses that are as similar as yours as possible, with as little p/u foam on top as possible...even maybe finding one that is Marshall coils with some latex on top...then you'll have your mattress!  Most of us don't have a clue when we started what we did and did not like inside of our mattresses and it can take a lot of trial and error to get it right.
There are some new developments at Leggett and Platt(make bedsprings), including a new Marshall type spring that is made for side-sleepers...I sure think it looks interesting.  Maybe they can steer you to a place that uses these coils in their beds. Bodyprints is the name of their new spring that is a Marshall coil.
http://www.lpbeddingcomponents.com/body-print.asp
Kait

Should have listened to you... after we went to the Marshall showroom we tried a latex at another place.  We like the feel and softness and bought into the "Latex doesn't get hot" opinions expressed all over the web.  Perhaps Latex doesn't get as hot as memory foam but we lasted one night on our 8" latex mattress and both woke up (what little we slept) with two words...Hot & Hard.  The hard we could fix but the hot..... We are returning it and going back to the pocket coil drawing board.   Too bad Marshall isn't making the kind of bed we got... tight top Tudor Delux  Lux Comfort at a fair price.  We paid $800 in 92. Now they want almost $2000 for a similiar bed. Any recommendations for a good pocket coil other than Marshall?   Sounds like Simmons is garbage...   Any help appreciated as we continue to sleep on our old bed. 

Anyway you can find out the gauge of coil and the coil count on your old Marshall coil mattress? If so, try to find something as close to that as possible. Google is your friend.