I got them at an online retailer. Extreme discount. I took a gamble as I thought it might not actually be linen at the price. Especially wide enough to make sheets. Don't forget it shrinks.

Worthwhile going to a store to feel all what's available, different weights, weaves etc. There might be a clothing or upholstery oriented place that has more useful. For textile-industry, do not expect meaningful online presences. A lot of fabric stores have a website from 1997. A few 200px photos of stacks of fabric, a list of random brand names, a phone number and an address. Call them and tell them you are interested in linen for drapery, sheets & upholstery and they will tell you if have have that kind of thing and if not where to go. But it is a touch and feel business so actually going is better.

Also the word "linen" is used to mean a lot of things. some of which contain no linen. you want "100% linen" and knowing about weights will help https://www.onlinefabricstore.com/makersmill/linen-fabric-product-guide/

I sew, and I wouldn't even bother looking for linen in your local fabric stores. Most everything is going to be mixed with rayon and too narrow for sheets. That's especially true if your local fabric store is Joann. I can recommend some online retailers, but my usual go-to in the past has been Ukrainian linen, and... uh, you know.

For your current sheets, it will take time for them to soften, but if you wash them on hot and tumble dry them, they'll soften faster. There are multiple types of linen, and there's a variety called softened linen where it's been basically been beaten to soften the linen fibers and simulate wear. Linen that hasn't been softened just hasn't gone through that process and will be scratchier.