Only 2 items, that if you really wanted to, you can purchase: boots and a watch. Honorable mention: gloves
Boots: If you do plan to get your own pair, I highly recommend you go to a shop that has them so you can try them on (just like all shoes really). At the VERY LEAST - if you must buy them online, ensure that that retailer allows returns for size exchanges. The last thing you want to have happen is order boots that turns out to not fit right to only realize they don't do returns (or they charge a fee for returns/exchanges). Boot sizes (just like sneakers) kinda very by manufacturer. You may be a Nike 9, but you could be a Rocky's 9.5. Speaking of which - there's 2 main kind of boots: you have your ultra light and minimalistic Nike boots and you have your heavy Rockys S2Vs. Nikes are great for rucking (since they are light) and garrison ... however extended or prolonged field time will quickly wear out your boots. Rockys are heavy, but super durable and last a long time with field use. Pros and cons.
Watches: If you don't have a watch - get one. You 100% need one, not only as a CDT, but also (and more importantly) as an Officer in the United States Army. Doesn't have to be anything super fancy or expensive; if you're short on funds your cheapo Walmart $20 special will do just fine as a CDT (until you actually have an income to get something a little more durable). As long as it tells the time and has a stopwatch and timer feature, you're good to go.
Gloves: I mention this, in part, dependent upon what program you go to --> specificaly what geographic region it's located in. If it gets cold in the winter, I'd recommend grabbing your own pair. The ones the Army will issue you are okay; they're bulky, but do get the job done. But you'll probably start hating yourself when you're running around in the woods during an FTX trying to plot points during land navigation with gloves that are big and bulky. Get whatever gloves you want so long as it's within regs.