where can i get the lvl 5/15/30/45 clothes/sets and weapons??

These drop from monsters as usual, but it's unlikely that you find them this way anytime soon. That's what players are for --- you can buy stuff from them :-)

but how much?

but how much?

Prices vary --- as an example (Aibatt server), assist rares tend to be expensive, acrobat items are cheap in comparison. Male merc (Petal, Comet, Pangril) is expensive, female merc (Flower, Cylos, Stitch) is cheap. Weapons and suits are the most expensive rares; you'll often find players wearing the set helmet/gloves/boots, but not the suits, and a random junk weapon.

As for weapons... it seems there's always a weapon with the same name as a set (there are definitely comet swords and miragle yoyos) --- these have absolutely nothing to do with the set, even though some players try to make it look like they are part of the set to skyrocket the price.

Usually, new players don't have the money to afford the stuff, even if I list it as "cheap". As a *very rough* guideline, lvl 15 rare gloves/helmet/boots are around 20K, plus/minus something based on class or gender. Curus/Sardine (assist set) marks the top end of the price range; I wouild put these at 30K max. Everything else, including Petal (male merc) has to be lower than that. Not too sure about the acrobat and Flower (female merc) sets; these "have to be" below 20K. Suits don't really have such easy pricing...

The more time you spend looking around, the higher the chance for something cheap, of course.

Of course, things are difficult if you don't know the prices --- players often put insane price tags on their stuff, especially in Flaris (aka "Newbie town"), but you can find a lot of overpriced stuff in Saint Morning as well.

And newbies are easily "double-scammed", talked into selling their PD4 for too low, and then buying some overpriced rare because now they have some money... myself, I do try to put stuff towards the upper end of the price range when I sell it, but I haven't tried "insane prices" just yet...

At the same time, Flaris also has a good chance of insanely cheap rares --- occasionally it seems people don't know what they are selling, so they offer rares on autoprice... well, thanks for the donation, it helps with getting my stick :-)

At higher levels, rare prices (depending on item, of course) *can* skyrocket --- my assist did :-) have some cash which went into the Flury outfit for the most part (Flury/Wedge = lvl 45 assist set = insanely expensive). She's still rebuilding cash to get the lvl 45 rare stick... and I'm not even thinking about Jewelry. Or buying a Ringmaster outfit for "formal occasions" (I'll probably be using Flury forever, but that also means I'll probably look like an assist forever...); I'll probably just collect it from random drops.
The rare lvl 45 sword is pretty expensive as well --- lots of mercs going Blade, and I've even seen (low level) Blades (dual-)wielding these.
In comparison, the lvl 45 yoyo is pretty cheap; people basically throw them at you (pun intended :-))