I cannot give you a definitive reply but can raise a couple of options. If you are looking for a kit containing all the O rings you would need to completelyreplace all rings in the rifle then you may find that the 'bay' is a good starting point, or maybe Knibbs. If you are only after a set of the most common ones that are replaced, then do you already know the sizes you want? If the answer is no, then a quick search on google or even the O ring data base on here should give you sizes and spec for HW100 rings. Individual rings can be a tad pricey but they get much cheaper as you buy multiples. Places like Simply Bearings are good. But then you have to weigh up whether you want to buy multiples of the rings as you might well end up approaching the cost of a complete kit. Only you can decide this. If you end up with too many rings it is possible you could sell the spares on here.
Cheers, Phil