To start your journey - a list of lumber yards can be found here: LINK. Prices vary widely - both day-to-day and between suppliers.
I have used both Central Hardwoods and Hardwood Lumber Company. One of our members @Lloyd_Plum also has his own mill in Plano and is a great resource. Great source for wood
You can also find inexpensive lumber from Facebook Market Place/Craigslist - make sure that it has been properly dried [you might invest in a moisture meter]. This can be the dodgiest place to purchase lumber.
Most lumber yards sell different surface types: rough, S2S, S3S, & S4S. Rough lumber will need to be surfaced on all sides, S2S has been through a jointer and surfaced on a face & edge, S3S has been through a jointer & planer and will need the fourth edges surfaced on the table saw, and S4S has been surfaced on all sides and just needs to be cut to length.
You can also buy different grades of lumber: LINK. The grade will also greatly affect the price you will pay.
As to what species to use, it really depends on what you are making, what appeals to you, and your budget. Poplar is relatively inexpensive, but on the softer side of most hardwoods and doesn’t have a pretty grain pattern - so it is typically used for painted projects or “unseen” structural pieces. Oak is much harder, takes stain fairly well, and has a pretty grain pattern. The three wood “kings” of cutting boards are cherry, hard maple, and walnut.
Finally, go get your “Green Dot” - it will open the “Member’s Only” section of the forum [create a post in the “Member Access (Green Dot)” category. Also note other member benefits can be found here: Just did my first visit to DMS - #3 by TSki