I've used and like "The O-Ring Store". I find their site easy to navigate and they have a good selection. Very pleased with their shipping and packaging. $5.00 flat shipping fee may seem a little high for a smaller purchase, but I haven't used anyone else to compare.
Just received some o-rings from them today, but for an EHPRO Kayfun Lite Plus V2, so I don't know if I can help you. For the EHPRO, the main body o-rings (4 needed) are 1 mm (Cross Section / Thickness) X 18 mm (Inside Diameter) although 17.5 or even 17 will work. Wouldn't go smaller.
The fill screw o-ring is 1.2 mm X 2 mm, although 1 mm X 2 mm appears to work fine (I'm using it now). The fill screw is actually 2.5 mm on the threaded region so 2 mm will stretch a little when putting it on. 1.2 mm X 2 mm may be hard to find however. Next time I order I'm going to try the 1.2 X 1.8 mm. I have the 1.2 X 2.5 mm and it's a little sloppy.
Did not size the two drip tip o-rings.
The o-ring embedded inside the outer top cap which fits around the chimney top cap appears to be 1.5 mm X 4.5 mm, but that's based on measuring the replacement o-ring that came with the kit as I haven't attempted to change that one yet.
I'm fairly confident that the o-ring material that came with the original is simply nitrile Buna-N 70. That is also what I purchased and it appears identical.
Looking at ads for the infinite, it appears you have extra parts that allow you to assemble a shorter tank. I don't have that option so can't help you there.