Hi bweaver,
This would depend on which of the Dreamfoam mattresses you were looking at. For some (such as the 13" gel memory foam") they provide you with information about a widely available mattress that has a similar “feel” (in this case the Tempurpedic Cloud Lux except they have said that their mattress is just a bit firmer).
In other cases (as JDMWDC mentioned) you would need to find a local mattress that had a similar design and materials which would approximate the one you were considering. I would also keep in mind though that any differences between them in terms of the materials in the mattress, the type of cover and quilting over the latex, and the softness/firmness of each layer in each mattress would also have an effect on how they compared both in terms of “feel” and how they performed in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) so I would treat local testing as an “approximation” unless every layer and component of the mattress was exactly the same in all respects.
There are some good local options available to you in post #2 here (Los Angeles) or in post #2 here (San Diego) that have good quality and value and that you could test in person if the lower risk of a local purchase or making the wrong choice is attractive to you or that you could use as an “approximation” for an online choice.
Phoenix