Right now it seems like the Bro-tek spacers are all out of stock, at least for my tractor. Bora spacers seem to be more available. I'm not in a hurry so I'm hoping Bro-tek gets back to making more. There may be other brands, but those are the 2 I looked into.
I have have added rear wheel spacers and and filled my rear tires with fluid and in my opinion this has made a very noticeable difference in my stability on a slope (as well as a reduction in the pucker factor). My unit has a FEL that I keep lowered as this also helps stability when moving on slopes. If I am doing a lot of loading with the FEL I have a box blade I carry of the 3pt and this really adds stability in keeping all 4 wheels on the ground.