Chest freezer

This is by far the cheapest, easiest and most efficient option. For private use or for less than 20 people per week it works great. It just becomes limited if youre running several sessions per week, with multiple sessions per day, then you need multiple chest freezers just to keep the ice baths icy.

Assuming the  projected numbers are looking favourable, you can buy a second hand chest freezer (200-400L) for $300-750. Silicon up the hole in the bottom (it will leak otherwise) and the internal corners (the edges come off every time for me but it doesnt seem to matter. If you have a leak you can usually trace it back to where it comes from).

What brand?

Different freezers and models freeze differently. It depends where their cooling components are in the walls. If you want a freezer for yourself at home, having a freezer that freezes from the bottom can be good, so you don't have to deal with breaking ice just to get in if you leave it on for too long. For public sessions, I like Fischer and Paykel freezers, as they usually freeze from the top. I break the ice up with an axe and people can't get enough!

Cleaning

For a personal chest freezer ice bath, you can get away with cleaning it every two weeks. After the initial few days it takes to cool down, just plugging it in for a few hours every few days should keep it cold enough. For public sessions, I empty the entire chest freezer, clean and refill  it daily. It uses a lot of water and can be time consuming (while the sauna itself takes 10 minutes to clean, if you're not careful you can feel like you've become the town pool-boy). 

Making it look nice

You can clad it if you like. Using some glue like liquid nails, you can put some timber strips on which you can pin on some cladding afterwards. I recommend against cladding the top as the hinges are not made for the extra weight. 

Proper ice bath

Pricy option. Hard to know how much margin the retailers are adding. Seems unreasonable. The only time I would consider this is for 100+ people per week. Otherwise you'll be spending half your income just paying off the ice bath which was suppose to only be an addition to the sauna anyway! 

Currently, you have the option to buy full set ups which have been imported from Australia or China or to have a crack at it yourself. The only problem is that this obscene margin has crossed the industry, while unbranded chiller pumps etc are hard to come by. 

Lake/sea water

No cleaning and prepping needed! The down side is that it's just not the same as a 0.2 degree submersion! Plus you usually have some distance to walk to get to the water, by which time you've cooled down in the wind and are struggling to decide if you should just walk back to the warm and safety of the sauna.

Blow up ice bath 

So impractical. I'm sorry I couldn't start with something more positive. Buying ice is heavy and expensive. Plus the amount of plastic the ice is wrapped in makes your conscience feel bad, let alone the inflatable plastic tub it is going in. Your self-esteem will be too low to run anymore sessions after all the climate shaming your customers will give you, as they come out of their favourite plastic-bag-free supermarket.  Anyway, after a number of these sessions you may as well have bought a chest freezer which would make endless ice for you and not melt away on your drive back from the petrol station. The only plus side with these is for people travelling and doing workshops, for example, who don't have the luxury of  slowly cooling water for several days.