I buy lots of the same styles I like (especially with things like running shoes - though I do vary color). Most of them are probably repping so they would just let the company know what they need

When I find something I really like, I go a little overboard.

I don't watch much tennis anymore, but I do watch golf, and sometimes I'd like to know where to buy some of their articles of clothing.

...can be bought through the PGA Superstore or Golf Galaxy/Dick's Sporting Goods (GG & Dick's are the same company.)
There are some players that forego clothing sponsorships, including a few big names. But mostly, the companies give those clothes to the players so people like you & me will see them & want them. (I'm an exception in that I don't wear golf shoes & I mostly wear Hawaiian or cabana shirts.
There are also websites devoted to selling the fashions designed for specific players. Like this one:
https://www.golfposer.com/players/rickie-fowler

But my guy does, and I'm always on the lookout for something to give him when a gift-giving occasion arises.

I've hinted numerous times that I'd like to play, but he golfs with his buddies three times a week, and I think that's enough for him. I don't want to intrude on his time with them, and I suspect they wouldn't really welcome his novice girlfriend tagging along anyway.

I think you'd be surprised at the reaction of golfers to a girlfriend who can actually hit the ball.
The big objection most guys have to the GF coming alone is that they are terrible novices, not that they're a GF.
Same thing with playing with somebody's kid. If they can hit it & know some etiquette, nobody I know would have a problem.
But, a raw beginner whi misses the ball, or huts in 10 feet over & over is frustrating because of all the waiting around.
If you can actually hit it like a weekend golfer, your BF might be willing to play one more time each week just to have that round with you!