I'm looking for a positive word for a person who always searches for and finds the best deals before making a purchase, no matter how small [duplicate]
The term that comes to mind is
penny pincher n. [One who is] extremely frugal in giving or spending money.
(American Heritage® Dictionary of the English Language, Fifth Edition. S.v. "penny pincher." Retrieved April 9 2016 from http://www.thefreedictionary.com/penny+pincher )
Your request for a "positive" word came after I'd made the 'penny pincher' answer, but doesn't invalidate it: 'penny pincher' is often used with a very positive sense, especially by people proudly describing themselves as such. Any word, of course, may be used with negative or pejorative force; words used negatively have negative connotations in a negative context of use.
I did overlook that you asked for a single word. 'Penny pincher' is often used in the closed (pennypincher) and hypenated (penny-pincher) forms:
Most common of late, in the Google Books corpora, is the hyphenated form.
However, 'penny-pincher' aside, 'thrifty' and 'economical' suggest themselves:
thrifty, adj.
1. Practicing or marked by the practice of thrift; wisely economical.
(American Heritage® Dictionary of the English Language, Fifth Edition. S.v. "thrifty." Retrieved April 9 2016 from http://www.thefreedictionary.com/thrifty )
economical adj.
3. a. Characterized by or tending to economy in the use of resources; not wasteful, sparing; (of a person) saving, thrifty.
["economical, adj.". OED Online. March 2016. Oxford University Press. http://www.oed.com/view/Entry/59385 (accessed April 09, 2016).]
While both 'thrifty' and 'economical' are adjectives, they work in your example sentence with a slight alteration:
My brother is so thrifty he got ....
The same alteration of your example sentence works for 'economical'. I favor 'thrifty' over 'economical' because 'thrifty' is a more thrifty word, and maybe because it rhymes with 'nifty'...maybe not.
However, neither 'thrifty' nor 'economical', nor other suggestions such as 'frugal' or 'sparing', carry the denotative force of 'extremely frugal, thrifty, economical' that is part of the definition of 'penny-pincher'. It is precisely that sense of extremity, perhaps, that leads some, particularly British ('penny-pincher' is a US colloquialism), speakers to feel that the word is only used pejoratively.
Consider bargain hunter.
She likes to hunt for bargains when she shops. - MW (From: Examples of BARGAIN in a sentence)
See also the show Bargain Hunters.
bargain-hound or frugal shopper.
Does it have to be a noun? If an adjective will do, canny or savvy
canny, defined by The Free Dictionary
Careful and shrewd, especially where one's own interests are concerned.
Cautious in spending money; frugal
canny from The Online Etymology Dictionary
1630s, Scottish and northern English formation from can in its sense of "know how to," + -y . "Knowing," hence, "careful."
"My brother is so canny -- he got a night in a 4-star hotel in Paris for $112...."
or
"My brother is so savvy -– he got a night in a 4-star hotel in Paris for $112..."
I've written the OP's sentence with a single word in the blank, but it would be more natural to say: "My brother is such a canny shopper...." or "My brother is so savvy about prices...."
Definition of savvy from The Free Dictionary
Well informed and perceptive; shrewd: savvy Washington insiders.
Knowledgeable or proficient. Often used in combination: tech-savvy; media-savvy.
The signature of a canny or savvy shopper (as opposed to, say, a cheapskate) is that they look not only at price, but at value, including the value of their own time. For example, the OP's brother might have rejected several hotels that were even less expensive because they were not within walking distance of the major sites and would have meant a lot of time spent in taxis or on Metro.