Snurge: Most sources I've seen regarding the Ideal sizer, recommend replacing the old pressure nut, with one from a 450/4500 sizer. As lube reservoir barrels are similar in inside diameter. Lyman part no.: 2990708. I have not yet had to do this, but makes sense. If in doubt, contact Lyman directly, for more precise information.
As far as o-rings, you can buy replacements at Canadian tire. Ordinary plumbing o-rings. Dimensions: 13/16" ID X 1-1/16" OD X 1/8" Wall. These are a perfect fit. At least, for the 450/4500.
Hope this helps.
Al
Edit: I may just replace the pressure nut, after all. The brass collar on the 45's pressure nut is prone to a fair bit of lube leakage. Annoying. I generally use the 45 with soft lube only(Bullets for bp cartridge revolver). So no issue with damage to press. Extensive use with hard lube could be problematic, though.