Try to obtain supplies directly from the source.
You sould be targetting niches which are less saturated by competition, you can conduct a manual market research of what is selling and what is not selling through goodbay and various other manual tools and then determine your profit/loss per product, remember to consider the eBay fees and PayPal fees.
As for your actual question, you can try many sites which are commonly used, but I prefer getting into personal contact with your supplier and let them know you are an eBay seller and they will be more than happy to assist you into growing your little eBay business bigger. Suppliers need to see you as a potential for them to grow in sales, most suppliers are middlemen, that's what you will commonly find, but if you do get hold of factory sales staff, this should set you up for the long run. If you have any further questions, just ask and I'll be happy to help.
One more pointer, the suppliers you find on everyday sites are most likely also going to be selling to hundreds of other eBay sellers just like you, so you need to be very careful and think right about choosing the right kind of supplier/dropshipper.