For small samples of Titanium I have found eBay to be convenient source. Sometimes the right alloy pops up. You can also try to find a used implant. Hospitals or orthopedics should be a good source. You can also try to contact a implant manufacturer to send you a sample for your research.
In case you are not inteested in certified materials, north ammerican alloys always is a good choice. Currently they have Ti-6-7 in stock. Please check