There not sold anymore, you either have to make them or like xtell use O-rings. Another option that works is to cut old toilet rolls and use them as sleeves or buy sleeves from a company like this.
As has been said if the gap bother you slide a few O rings down the tube. The element does not seal against the tube but with the base where the large square rubber ring is.
One word of advise is when installing a TP roll and assembling the filter do not press the roll all the way into the housing. Push it just far enough to clear the flange. When you install the housing onto the base it will push the filter in to the point it will be properly seated against the housing and you will not have to worry about oil bypassing the filter.
Anyone have a source for the o rings to adapt to larger filter tubes?
I have two of the filter set-ups. A ton of old school tp. A bag of lid o-rings. No filter sleeves.
I use Viton ETP O Rings. I got them at a shop that carries fluid connectors for off road construction equipment. I just brought in my Frantz filter mount and the guy matched up the o rings to the center shaft. They are rated from -15F to over +400F and good for use with petroleum products. You could also use round Silicone o-rings. They also come in many sizes and are high and low temperature resistant.