Not too sure how good or reliable they are, but I have seen quite a few Square D and Cutler Hammer XO - style breakers for sale on eBay. Like I say, I'm not sure how great an idea it is to buy something like this on eBay.
Regarding the lack of a main breaker, I bought an old Square D XO panel and breakers on eBay some time back (to add to my collection). The branch breakers were XO, but there was also a 2 pole 70 amp "multibreaker" style main installed. An unusual combination, but in excellent condition.