Combi rear spring replacment ideas and options

Hello.

My 03 fabia combi is in need of new rear springs. 

I carry a bit of weight around, tools and materials for work, I use a clamp on bike rack and tow up to 750kg often.

A search of ebay has only brought up one or two suppliers and they seem to be just like for like replacement.

I was hopping I could track down some heavy duty springs and would be grateful for any ideas or thoughts as to the options open to me.

Thanks

Jon