Longstreth.com - This company is based out of Pennsylvania and has a huge selection of gear; turf shoes, sticks, shin guards etc. 

Dick's Sporting Goods - Unfortunately, our local location does not carry actual field hockey gear in-store. I was told that some San Diego locations do carry gear. 

We can find out more information. 

Mouth Guards, Shin Guards, and Goggle rules straight from the National Federation of State High School Associations (NFHS) Rule Book.

Article 1  All Players shall wear mouth protectors.  A tooth protector shall be of any readily visible color, and may not be completely white or completely clear.  A tooth and mouth protector (intraoral) shall include occlusal (protecting and separating the biting surfaces) and labial (protecting the teeth and supporting structures) portions and shall cover the posterior teeth with adequate thickness.  It is recommended that the protector be:
a.  Properly fitted, protecting the anterior (upper) dental arch; and 
b.  Constructed from a model made from an impression of the individual's teeth; or
c.  Constructed and fitted to the individual by impressing the teeth into the tooth and mouth protector itself.

Article 2   All field players shall wear shin guards made of plastic, foam rubber or fiberglass, which shall cover the front of the leg from the ankle to just below the knee.  

Article 5  All field players shall wear eye protection that shall be permanently labeled with the ASTM 2713 standard for field hockey at the time of manufacture.  
   ****RULE CHANGE AS OF JULY 6, 2020****
In response to longstanding concerns regarding the wearing of goggles in high school field hockey, the NFHS Board of Directors approved a change in the eyewear rule that will take effect with the upcoming 2020 season.

Rule 1-6-5 in the NFHS Field Hockey Rules Book, which currently states that “All field players shall wear eye protection that met the ASTM standard for field hockey (2713) at the time of manufacture,” will now state that “Goggles may be worn by all field hockey players.”     
  ***This rule is being looked into based on District and League meetings***