Lexmark printers will take generic cartridges, you just have to make sure you buy the correct generic cartridge. I've had two lexmarks in the past (one a desktop printer, the other an all-in-one) - not top quality printing, but suitable for homework or personal letters.
I used to buy our generic cartridges at radio shack, and they also sold ink refill kits for the lexmark printer (calidad solo brand). There is a trick to getting the generic cartridges to re-register in the printer software to display as full, though most generic cartridge packs come with the instructions.
I know that wal-mart (at least ours did) sold the generic cartridges for a while, as did a couple of other sources (Staples was one). Lexmark cartridges weren't any more expensive than HP or Epson - for the most part, ink cartridges for most printer models fall within a few dollars of each other. Locally, I find the HP cartridges more expensive than the lexmark were, and the Epson is more expensive than both.
Let's face it, you can't buy ink cartridges of any kind "anywhere", you can only buy them in shops that sell printer ink. Quite possibly the rep had no idea what stores were local to you, or what stores carry generic cartridges for all brands.