Suggested setup for your best friend?

I make an effort to support US businesses, so the first choices are the obvious ones Simple-Shot.com and PocketPredator (Bill and Daranda are big supporters of this forum)

As a beginner, Walmart is a great place for ammo, band material, rotary cutters, self-healing mats, and rubber bands. You can find Daisy 5/16 and 3/8 steel shot in the hunting section - not the best price but instant gratification and one-stop shopping. In the exercise section, there are SPRI resistance bands, you get a pack of 3 for $10 - look for the latex warning on the package! Not the best bands in the world, but great to experiment with and to practice your band cutting technique. Rotary cutters and self-healing mats are in the sewing section. Rubber bands are in office supply if you want to try braided or chained rubber bands. Rubber bands make very sturdy band ties, or just cut some strips from the resistance bands.

Paracord and jute twine are in the home improvement section to make wrist slings and lanyards.

Pocket Predator, Simple Shot, Double L Slingshot Supplies and Sling Tech. All US based companies. Very helpful and offer great service. A quick search will point you to their sites. I won't say one is better than the other, but a good (at least initial) place for elastic is Sling Tech (in Florida BTW) because they sell short tester lengths. You can buy 5 or 6 different types of elastic for the price of a single roll. Makes finding what works for you a lot easier and much less expensive. For steel check ebay for PGN and BC Trade. I personally use PGN because they generally have better pricing. As I've said in the past, there is no such thing as "slingshot ammo" so don't pay a premium for bearings labeled as such. A search for utility grade steel ball bearings will sometimes result in better prices.

A side note on ammo... Besides searching for slingshot ammo and utility grade, get creative in your searches. Like "round steel balls" etc. Sometimes people post things without any real use in mind. Include "catapult" in your searches as well. Also, when searching "slingshot ammo", make sure you check all the pages. You can find good deals buried deep. I found an excellent deal on used Pachinko balls, which happen to be 7/16 (11mm). They're cool because they have Japanese writing and symbols on them.