Flaming Dagger. (Also not unique but I don't quite know where to get more consistently.)
You can buy more from Vivianne, the witch who gave you the first one, in the starting forest. She can also sell a bunch of special items and train you in black magic, including a unique spell you can't learn from Ivris. Anyways, yeah, I want more equipment space too.
Consider how your Champion fights and perhaps keeping only the items pertinent to that playstyle. I see a bucketload of space you could free up with that alone.
Is he or she a warrior? Sell the Witch gear and other magic/sexiness based equipment, are they a mage? Then sell the warrior equipment. Either way you suddenly have a lot more space and some electrum in the bargain.
The thing about that is that that requires players to not have a hoarder mindset, especially in regards to rare/unique equipment. Once you sell an item, it's gone, and if that's the only copy or it's hard to gain more, then you don't want to get rid of them, even if you likely won't use them. It probably is best to sell the items you almost certainly won't use, but like, I don't think like that. I think like "ooh, shiny" and collect it like a crow, regardless of if I would actually use it. It isn't helped by how later equipment is supposed to be sidegrades rather than straight upgrades (primarily), and by how you can respec, learn abilities from other classes, and swap your class entirely. Since early equipment is intended to be still just as useable later on in the game, you don't have the pipeflow of TiTS in regards to weapons, where new ones come in and you sell the old ones because they're just worse. This is good if you have a small pool of equipment, but at this point, with all of these unique equipments and rare consumables, with most of the equipments not being
completely out of date, you're incentivized to keep them yet punished by the storage/inventory cap.
While the easiest solution would be to sell off some of that gear that's only sitting in your inventory out of a sense of completion, it will also help you to note that there are more slots. If you can only store thirty items then you've missed one of the currently available storage upgrades (Garth, Wayfort, Senja).
Senja can give you a storage upgrade? I didn't know that.