I'm pretty sure you can buy blank Minkus pages to deal with that problem. Or have a print shop make up a batch of pages in that size. Alternately, you can look on
ebay for Minkus pages to fill in the gaps in your album. Sometimes pages for specific countries are listed for sale, usually with stamps on them. That would be the easiest and best looking solution. It might take quite awhile to find them, though. One of the hazards of buying used albums is that sometimes pages are missing, unfortunately.