It's good to see more discussion here after a long break from July to December!
I have been trying this film (Aviphot; whether from Maco, Wittner, Lomo or others) and to understand its yellowing ever since the end of Kodachrome was announced. After 6 years and at least 60 rolls here is what I can offer about avoiding yellowing:
1/ The film must be well stored by the manufacturer, the retailer, the distributor and the user. Oh, and the delivery company must not have kept it stored in a hot postal warehouse in transit!
2/ The 35mm film must be loaded in low light to avoid light piping. I have never had a problem with 120 film!
3/ Be aware that the film is very sensitive to warm light - I have shot rolls that I thought had gone yellow, just because I took the photographs on a sunny afternoon, and the film emphasised the sunlight.
4/ It must be developed as soon as possible after it has been shot! I have never seen this mentioned anywhere, but I have found very clearly that film which was stored well and developed within 24 hours is perfect, with no yellowing, whereas the same film developed 4 days later is noticeably yellow, and if developed after more than a week it looks dreadful - unless you want a yellow cast.
I think it is this last problem, number 4, that gets most people. The first roll you shoot, you will probably take all in one session, and you will get it processed as soon as you can, to see the results. They will look good if you followed points 1 to 3. Then, later, you will shoot a roll over a few days, and you will have it processed in less of a hurry, and you will think that something has gone wrong, or that the film has gone yellow with age. I have used this film well past its "process by" date and if it was well stored and processed very soon, it was just fine.
Back to the question of where to buy it - I have had very good results with film bought direct from Wittner, who clearly store it well and ship it quickly. I have had mixed results with Maco (Rolleichrome 200), depending on how it was stored. Fresh film is fine whether bought online or from a dealer, but some dealers keep it on the shelves in their nice warm store for months - I have had some very poor results with Lomography X-Pro 200, which the dealer told me they just keep on the shelf till it is sold.
I would like to compare these experiences with other people's, so I have started a new thread just on the subject of avoiding the yellowing of Aviphot.