By asking an artisan dwarf (so lvl 20 minimum) to Crystalise any D grade drops you migh have and won't use or buy them from the private stores (stores set up by fellow players).
Should you attempt an over enchant of your D grade weapon and break it, it will decompose into D grade crystals as well.
2. Setup a buy shop overnight in Giran buying them for 550 adena (price may be lower depending on the market) on all the money that you don't need to spend urgently on something else.
3. Put into your warehouse the amount of crystals you are going to spend in the nearest future.
4. Sell the rest in a sell shop in Giran for 650 adena (price may be higher depending on the market) over the next night.
don't bother using your summon, if that's what you're thinking.
Not everyone plays just to be the uberest player ever. Let the OP have some fun and test whether they want to become a summoner or a nuker. Being a summoner takes a liot of practice, skill and patience. At least if you want to do it right, I still can't, which is why my summoner is still level 31.
Gustin....still can not find a D-Grade crystal or someone to crush a D-grade item....help
Where are you looking? I usually see at least a couple of shops selling D crystals between Dion and Giran every night. C Crystals are almost as common, B Crystals are a bit more rare and A crystals are definately rare. (Rare as in finding the raw Crystals for sale on a afk shop.)
If nothing else, try checking the racetrack for some. (Giran/Dion is better, but it can't hurt.)