So some advice on your question. Let's start with some help on guild stores for new players.

Best guild store locations: the most expensive locations tend to bring the most stocked stores, which can be found in Elden root, Rawlkha, Wayrest and Mounrhold.

Searching for gear: you can't look up a specific set, so the best you can do is use appropriate filters. For armor, I set the character level range, armor weight (drop sets mostly come in One weight), then I might search a specific piece and trait (i.e., chest and divines). This can narrow things down to find what you want piece by piece.

Low level gear isn't generally in guild stores: that all said, people don't list low level gear anymore. It's not worth enough to take up inventory slots, and you can only pick it up with a character of that level...and most people don't farm w a low level toon to get gear to sell. Also, gear is only good within about five levels of the character's level...by the time you farm/collect a full set of lvl 20 stuff you might be lvl 25 and need to throw it all away.

Use craft sets: your best bet is to use crafted sets while you're leveling. You can't make jewelry, but that's ok. Wearing a 5 piece set then 3-4 pieces of another set will easily suffice for low level play. Two craft sets to consider are Magnus and Seducer. This is a 3 and a 4 trait set...you can complete this research in less than a week. Learn each piece in divines, training, infused, then whatever else you have. You can buy pieces to research in guild stores for about 500ish each piece, or put out a zone chat and ask someone to make you a light divines set for research. People are usually happy to do it for cheap/free. Guildies might help too.

As an alternative you can find a crafter to make it for you, but honestly you'll be better off just doing the research and making your own. Then you can upgrade whenever you like. If you do decide to do research, go ahead and do up to 6 traits in all the stuff you use. Being able to craft six trait sets is useful well into vet.

The crafted set route isn't quite as powerful as Mother's Sorrow, so mahbemypunwant to farm/grind it. But the crafted set configuration is about 95% as powerful, and saves hours and hours of looking and relooking for gear.

The time to for sure start looking for drop sets like Mothers Sorrow is when you get past vet and 160 champion points. (It doesn't take as long as it might sound). C160 gear is prolific in guild stores, and that's where you can start accumulating sets that will last long enough to be worth the effort.