With breeches the size is your waist measurement. I'm a size 4 in most stores (Gap, Banana Republic, Limited, etc) and I like my breeches to fit just a touch loose. Depending on the brand I'm about a size 26 but I like to buy 28s. In ariats I'm more of a 26. Being 5"8 and only 125 you may need to buy a "long" size if they offer one.
Not sure about dressage show shirts but with ratcatchers you're about one size up from your bra size. So I wear a 34 bra, and my shirts are a 36. Again, depending on the brands. Some fit roomier then others. Tailored Sportsman also goes by regular dress sizes.
I'm surprised you can't find anything off of ebay. I'd just keep looking. I can usually find several items there and you aren't a really unique size.
I've never bought these specific RS breeches but I have owned the knee patch kind. They're ok. I'd wear them to a schooling show but I found they didn't last as long as some higher quality pairs. Have you looked through their clearance section? for the same price you might be able to find a better quality for the same price. Since breeches get more wear and tear I tend to spend more money on breeches then shirts.