Reddit mentions of TuffRider Men's Regular Cotton Full Seat Breech, Black, 34

Ok. As promised, I return:

Here's a pair of decent budget breeches. They fit true to size, and they'll hold up to light use:

https://www.amazon.com/TuffRider-Regular-Cotton-Breech-Black/dp/B002HUT6KQ/ref=sr_1_3?crid=1G1BX4V80NMXP&keywords=tuffrider+full+seat+breeches+for+men&qid=1562903407&s=gateway&sprefix=tuff+rider+full%2Caps%2C333&sr=8-3

Here is a pair of nice pleated breeches. If you feel nervous about wearing tight pants in public, these are a good option:

https://thepaddockinc.com/products/horze-grand-prix-mens-extend-full-seat-breeches-with-pleats

Here's a good pair of heavily discounted high end breeches. These are really overkill for what you actually need, but they look nice, they're well made, and they are comfortable. Order a size up from what you'd normally wear, because these are sized for slim European men:

http://www.dressageextensions.com/cavallo-mens-colino-grip/p/59756/

You will want a pair of breeches because they will keep your male parts up and out of the way. A pair of compression shorts will be a wise investment as well.

As for helmets, get what feels comfortable, but try not to go for either the most expensive option or the cheapest option. You want it to feel snug but not pain inducing. Don't be afraid to try a few different models. Tipperary, Charles Owen, and OneK are all good brands with well designed helmets.

You don't need expensive boots right now. Get a pair of Ariat men's paddock boots and maybe a pair of half-chaps. I'll send you links to products if you need help finding them.

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For your right now instructor, look for somebody with a lesson program and school horses. This does not need to be a dressage trainer specifically. Event barns can be great places for beginner lessons. Ask what the rates are for lessons on school horses for absolute beginners. Expect to pay between $60 and $100 per lesson.

As I said earlier, explain your goals and that you are a complete beginner. You'll probably feel awkward at first and that's fine. You won't be amazing at this when you start out and that's OK.