4uconnector.com has an amazing selection of connectors, including board-to-board. For standard orders they have minimum quantities of 100 or more, but they also have an option to order samples that appears if you create an account. The total number of samples is unlimited (you do pay for the samples and shipping), but they have a max sample quantity per part, per order. I've ordered a lot of samples from them to stock my parts box. Even factoring in the shipping, many connectors were much cheaper through them than standard suppliers. If you can deal with the slow international shipping, check them out.