The Table 4-1 in DMG P.105 says "If paid to create a specific item, use item prices and working times instead." and this sentence is applied for Alchemist and Mason/craftperson. So your PCs just pay the cost of non-magic items they want. That is usually just equal to buy items in a city or town.
As there is no true magic item craftperson in the table, it will be a job for more detailed NPCs, ones with actual levels in spellcasting class (though alchemists must be some kind of spellcasters due to Craft Alchemy rule, I guess most of them are simply 1st-level Adepts at best.).
You can use the ready-made city setting in supplements and modules to see what kind of spell casters are in a certain city (say, Theodra Goldenbrow and Immerstal the Red in Brindol in Red Hand of Doom).
You can also follow the WORLD-BUILDING rule in DMG section 5 and roll randomly to see what kind of NPC spellcasters are in a certain community. The rule will gives you information regarding class and level. Then you can give them whatever spells known and feats you feel to be appropriate.
Or, you can just make up some special NPC item makers of your own.
In any of the above cases, they will usually "sell" their magic items at usual market price. The benefit of consulting on such NPCs is that PCs may (or may not) buy magic items beyond that community's GP limit. Though they must wait for appropriate period (1 day/1,000 gp).
As a DM, you may give PCs some "discount" as a, say, reward for a certain task or adventure. But usually, the existence of such NPCs should not be used for getting magic items cheaper.
