How to replace rubber door seals Bosch Fridge Freezer?

i have an old Bosch Fridge Freezer model KGV3104GB03, which has knackered torn rubber seals on both the fridge door and freezer door(freezer has to work overtime trying to keep cold,so ice is inches thick!+fridge doesnt seal tight so is def not keeping cool enough) I cant afford a new one, so Ive checked replacement Bosch spare parts site- couldnt find seals for my model + the ones they had were also very expensive! Is there a cheaper option, say using non Bosch brand type seals for instance? or Ebay etc? and is it a do-able diy job in the first instance? Cheers in advance for any advice you can offer :o)