With the very explicit rules for weapons talking about installing and removing modifications, and the distinct lack of those rules for armor (for at least two editions now, I'm not sure about pre-4th), I have always taken that to indicate that armor modifications aren't nearly as easy to simply add to armor. A few things might be simple to install, but most of the modifications are factory/production-level modifications. For instance, making the armor Fire-resistant, or enhanced for nonconductivity is likely something done when the armor is being made, not added later.
The way I like to handle it in my games is two-fold. The player (character) has an option, either attempt to find the armor they are looking for with the modifications they want already integrated, or look for the armor and specifically tailor the modifications that they want. The former is faster, but harder to accomplish, while the latter is likely to take longer but more reliable.
Finding armor with the modifications that you are specifically looking for is like a normal availability test, except that the modifications' availability is rolled like a Teamwork test for the base armor's availability (or vice versa whichever is higher). And delivery time is based on the sum total of all the costs. So an Armor Jacket (Avail 2) with Nonconductivity (Avail 6) would teamwork eachother, resulting on average with an availability of around 7, with a delivery time of 2 days. Not so difficult for simple combinations (and base armors), but starts to be significantly more complicated with more exotic combinations. This method is also more likely to turn up odd combinations or GM created set-ups as you are looking for goods "off-the-rack".
Getting the armor custom fitted is the longer process. With this, each part of the armor is rolled for availability and deliver in sequence. You don't get the final product until all of the availability tests are complete. Effectively with this method, the roll for the armor modifications' availability is the process of having that modification specifically installed in whichever armor you are selecting. In this situation, delivery time would be based on the total time it takes to complete the availability/delivery times for each piece of gear/modification. This method is easier, as the avialabilities aren't teamworking each other, but longer, because the delivery times add together rather than being based on the single, total price.
Some GM fiat may be necessary to keep either of these methods balanced. For instance that part with the availability teamwork on low-availability armors.
