It's hard to find the right filters -- on Amazon or elsewhere. Non-Unicel filters from Pleatco or other Asian sources are being sold as 'equal' filters, and the descriptions include the word "Unicel", like "for Unicel model C-4607".

Here are the right links:
Unicel C-4607 Replacement Filter Cartridge (Easy Set Size A or C) @ Amazon
Unicel C-5315 Replacement Filter Cartridge for 15 Square Foot Intex B Filter @ Amazon
Currently, the 'A' model is not available from Amazon directly, but some 3rd party sellers have it at similar prices.