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In 2013, they redesigned the Sonic Screwdriver, so the newer models need a different type of cartridge. After about 2020, since everybody has the newer design, all the 2007 model cartridges go on clearance sales, and the prices drop a great deal. However, after 2033, 2007-model cartridges are discontinued, so the prices rise on them again as they become sought-after antiques. In 2055, a record is set on Neo-eBay for highest auction price on ink cartridges, with a price of 600,000 Adjusted Rupees as the Buy-It-Now option (the auction winner was not disclosed).

So, your best bet is to go to about 2022 or so, after the XMas sales, and root around in Clearance bins to see if you can find any for cheap.

Of course you can mod you screwdriver, you just need to download the plans (free) into your nanobot mill, mod the plans, and let your nanos do the rest. However, since you asked that question, I'm going to guess that you don't have a nanobot mill or don't know how to program it, so I would say to take it to a good atomsmith in the Interzone and have them do it for you - not as cheap, but it should work.

Please excuse my mangled verb tenses - English is most unsuited to the type of temporal engineering I describe.