Is there any strategy to layering the toppers? In other words, what is a good combo? I'm not really sure how to purchase them since I can only get them online. Are they returnable if we don't like them? Thanks. Sorry for the all the newbie questions.
There's no real stategy to layering toppers. Comfort is very subjective. Generally though you would go from firmest on the bottom to softest on the top, but some people mix it up with a fimer layer over a softer one. I like mixing different kinds of foam like latex, MF and HR to really fine tune the level of comfort and support.
I would suggest starting with just one topper. Over springs you would probably want something in the 22 (soft) to 28 (medium) range depending on your preference. Just a hunch, but I think you'd probably like it more on the soft side. If you're under 200lbs don't go over 2" of latex. I prefer working with 1" increments, but I'm a lightweight and too much foam undermines support for me.
Unfortunately it's really a matter of experimentation, and yes, you may have to exchange things before you get it right. Call a few vendors, tell them what you're trying to achieve with the topper and see what they suggest for you. I think foambymail.com allows you to return anything you're not happy with. SleepEZ charges a restocking fee for mattresses, but they may not accept returns on toppers at all. I think foamorder.com also accepts returns.
I don't know where you live but sometimes shops that carry upholstery supplies stock latex foam in mattress sizes. Check your phone book.