ok, well I can give you some tips on shopping
I am really picky, and can spend weeks searching before actually buying.. lol
First where to buy?
Obviously, online is cheaper than tack shops, but online has a variety of options too
and so on, are bigger company and can be more pricey, but have bigger choices
Some tack shop do online sales too, which sometimes is nice and secure, and cheaper
You can also go for craigslist, ebay, or even this forum, but you gotta put more trust into the seller than bigger company which have big reputation, so much more trustworthy
It also depends on if you want to buy new or open to used products
Now, second point, look for reviews, real people, with real experiences will have better opinions that can help you make the choice more than the description given from the company.
Know brand names, there are better quality,more durable brand, but will be probably more expensive out there, but there are also some really good brand that some people don't know. Example: I import a leather conditioner from France that people here barely know, I do not think I came across someone who knew it, but it is by far the best leather conditioner ever ! My saddle, half chaps and boots are 6 years + and still going with this product.
Trust your entourage, if you know people in the horse industry that have been there longer than you, they probably can help you decide
Anyway, this was more of a shopping help, than specially for the girth and half pad, but this can help you in the futur too