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I don't use anything real expensive either. Currently I've been using Minwax (a trade name here in the states) Wood Finish. A quart ran about $6.00 USD. I've used it a lot and I still have about a half of a can.
Any of the Varathane type wood finishes of whatever brand should work fine - experiment on scrap! One thing I have found is that the thinner types like the Minwax can seep completely through the leather, especially in the thinner weights. On a hunter type that's not a problem but it is something to consider when making a fancy sheath.
Another good sealer is any of the paste car waxes that use carnuba in them. Melt them at a low heat and brush it on. I haven't tried them but the liquid waxes that use carnuba should also work fine.
Another oldy but goody is spar varnish, which is relatively inexpensive and should be available at any boat shop.
The final one I have used and the one I use on most of my frontier stuff is pine pitch - I see you live in Norway but don't know if you are in the city or country. If in the country or if you have place available to you - take a look around any of the evergreen trees and where they have been damaged the pitch will have oozed out. Scrape this off and take it home. Melt it at low heat (it is highly flammable so be careful - I use an old crockpot, but an electric hotplate works good too.) Skim off the dross (garbage) that floats to the top and then let it cool completely. Grind it up to a fine dust by pounding on it or use an old food blender. Sift out any lumps and soak the dust in alcohol or acetone. It should be about the same consistency as your shellac mix. It too is a bit messy but it sure smells good.
I test all sealers by sealing and then CAREFULLY filling a mockup sheath with water after gluing and sewing. If the water leaches trhough the sealer find something else.