Since I could not find any cubes of grids that I see in pictures here in my country, I bought metal shelves a used zip-ties to build a cage. I used grids for the ramps and for the 2nd floor - those I found online and they are "fence" grids for outdoors for rabbits. Since they are not so sturdy, it would be very difficult to build a cage using them.
Tkank you for the comment. I wrote it here several times, so I am going to repeat myself. This is the only type of food sold for guinea pigs in my country. It is a very small portion of their diet. The biggest part is plenty of hay, everyday they get fresh vegetables and 3 times per week a fruit. They are not obese or sick and not one during my whole life died from bad diet. I looked for pellets, but I could not find it in any pet store and having them shipped from UK or other country is too expensive for me.
Maybe you could go without pellets and increase your veggies to replace them? Some people don't even use pellets. It would be healthier not to feed them that mix at all because it doesn't serve any nutritional purpose; just sugar.