Alexandrines typically weigh 200-300 (say, average is 250+) grams at least... And probably eat 20+ grams of pellets per day... depending upon the amount of other foods it eats, it may eat less.
If I have my calculations correct, a 10kg bag would last nearly 2 years... and it's often recommended to use the pellets up within 3 months of opening the bag... (it's possible for pellets to go rancid) so I've had to agree with henpecked, a 1-2 kg bag would be ideal.
Hard to suggest pellets when we don't know where you live. For example, Vetafarm, an Australian Pellet, I haven't seen being sold out of Australia or New Zealand. Harrison's and Kaytee (two American brands - one high end and one low end) can be bought in other countries, and there are also people willing to ship these brands out of country. However, other American brands such as Zupreem, TOP's, Mazuri, Pretty Bird, etc I haven't seen being sold outside of North America unless someone buys it to import it to their company.
ATM, I don't recall all the brands available in the UK and am unaware of the ones available in Africa, China or Japan...