If you can't or won't get commercial developers where you live, or if you just want to mix your own developers, you have to get some basic chemicals.

For Caffenol, it's easy. You need cheap instant coffe, ascorbic acid and washing soda.
If you want to develop high ISO films, you would like to have some potassium bromide (KBr) to reduce fog.

Other and more advanced developers contains more chemicals, both to make them better and to make them keep longer.

For Xtol-clones and other ascorbic developers, you will need ascorbic acid, sodium ascorbate, Dimezone-S (a phenidone variant), sodium sulfite, borax, maybe lye, Sodium hydroxide (NaOH) and some chelating agent to increase shelf life. The best (IMHO) chelating agent is Dequest 2010, also sold as Photoplex-2.

What about Rodinal? Use Parodinal. Then you need some paracetamol tablets (tylenol) in addition to the lye (NaOH) and sodium sulfite

With this stash of chemicals at hand, you can mix a lot of slightly different developers.

So, where do I get all this stuff?

Washing soda and coffe is easy to find. I don't have to explain where to get that.
Do not use baking soda. It's NOT the same.

Ascorbic acid may be found in the spice department in your grocery shop. If not, you have to buy it on the net.

This is one source for ascorbic acid:
www.ebay.co.uk/itm/CLASSIKOOL-ASCORBIC-ACID-HIGHEST-GRADE-100-VITAMIN-C-PHARMA-GRADE-PURE-POWDER-/360658752444?pt=UK_Health_Beauty_Vitamins_Supplements&var=&hash=item53f8efd3bc

If you want sodium ascorbate,
¨http://www.ebay.co.uk/itm/250g-of-100-Pure-Sodium-Ascorbate-Powder-Vitamin-C-/320775069746?pt=UK_Health_Beauty_Vitamins_Supplements&hash=item4aafaf2032

KBr:
www.ebay.co.uk/itm/POTASSIUM-BROMIDE-250-G-KBr-PURITY-99-5-/200925401718?pt=UK_BOI_Medical_Lab_Equipment_Lab_Supplies_ET&hash=item2ec8165276

Sodium sulfite.
www.ebay.co.uk/itm/Sodium-Sulfite-PHOTO-more-Chemicals-for-less-1-Lb-/350240956675?pt=LH_DefaultDomain_0&hash=item518bfce103

Borax may be found amongst washing powers etc in you nearest store, but it can also be bought on the net.
www.ebay.co.uk/itm/BORAX-Sodium-Tetraborate-Decahydrate-High-Purity-Technical-Grade-/290925810679?pt=UK_BOI_Medical_Lab_Equipment_Lab_Supplies_ET&var=&hash=item43bc8793f7

Lye, Sodium hyroxide (NaOH) may be found in stores around. It is sold as caustic soda in Norway.
www.ebay.co.uk/itm/Sodium-Hydroxide-NaOH-500-g-Prilled-/130913441607?pt=UK_BOI_Medical_Lab_Equipment_Lab_Supplies_ET&hash=item1e7b0c9747

Dimezone-S
www.digitaltruth.com/store/cart/Dimezone-S-100g.html

100g is enough for a lifetime. You need 0.15g - 0.20g to mix a liter of developer. In this you can develop 10-15 films.

So for the chelating agent. Get some photoplex-2 from Suvatlar.
www.moersch-photochemie.de/content/rohchemie
Click on the link "Preisliste Rohchemie" to get a PDF with a list of chemicals and prices.
He has a lot more in store than photoplex-2!
He takes payment through paypal, just as easy as e-bay.

If you want other chemicals, a quick search on the net will find both metol, hydroquinone and other developing agents.
What is more difficult to find is "balanced alkali", sodium metaborate, but we can mix that from sodium hydroxide and borax, so that problem is easily solved.