Hi Ginger,
Most retailers or manufacturers don’t allow returns or exchanges on toppers and the only members I know that have a topper return or exchange policy are Brooklyn Bedding, SleepEz, and Select Foam. Brooklyn Bedding allows one exchange and SleepEz and Select Foam have a return policy less shipping costs.
None of the members here sell polyurethane layers that I’m aware of but there are usually local foam shops that sell them in a range of quality/density levels.
Post #4 here also has a number of other online sources for toppers including latex and polyfoam. Some of them (such as SLAB) also have return policies but you would need to check their site or call them for the details because I don’t know them off the top of my head.
Phoenix