Hi Becca, 
 
Thanks for your question.
 
Firstly, I would question what type of base the mattress is currently on. If it is on a sprung base that gives the mattress a softer feel then this could be where the issue is ocurring. If this is the case then it might be a lot easier to replace the base for a solid platform top; Something like our own Contemporary Platform Divan might be absolutely ideal for this. 
 

 
What I'd advise after this is to check a company called Soak and Sleep online. They have a much wider range of toppers than we do and they do describe some of them as Firm. My advise here would be that this description more than likely relates to the density of the topper or the materials. Any topper will always add an extra comfort layer so this will always in some way soften down the feel.
 
As such, this is the reason I've queried the base first as this is usually easier to rectify the issue. 
 
I hope this has answered your questions fully but if you do have any further queries please don't hesitate to contact our office on 0161 437 4419. Our office is open Monday to Friday 8am to 8pm and Saturday and Sunday 10am to 4pm. 
 
Kind Regards, Lee.