I recently re-edited and made some BIG corrections, that you may notice when you re-read this:
Using the Pool Calculator, type the size of your pool (in gallons), then in the Borates section (this is what I can come up with so far) type "0" in the first box and "50" ppm in the second. This should tell you how many ounces of borate you must add, and then divide that total amount by 76 ounces (which is one box of borax, and I'm going by the "20 mule team borax" method as I haven't done any other). This will tell you approximately how many boxes of borax you need. For my 12,000 gallon pool I need about 9 boxes to reach 50 ppm. Now, divide 50 by "amount of boxes you need" and you will get the amount of ppm that is added to your pool for one box of borax. For my pool I get 5.6 ppm of borates per box.
Next, get the max depth of your pool in inches (it's easier to calculate percentage with inches), and make an estimate of how much water your pool would lose in a one month period. Different pumps and filters pump out different amounts of water per sec./min. My pool loses about 16 inches of water per month from backwashing once a week, which is 22% of pool water loss.
Finally, if you can remember the last time you recorded the amount of borates in ppm or the first time you added borates to your pool when it had 0 ppm of borates, you will be able to proceed with this method. Now, in the Pool Calculator, in the first box of the "Borate" section type the amount you remembered and in the second box 1 ppm less each time until you get to the amount of pool water that needs to be lost/replaced, which is what you calculated as your monthly water loss/replacement (on the right side of the pool calculator it tells you the percentage of the water loss/replacement). The number on the second box should give (in the Pool Calculator equation) the amount of water (in %) that needs to be replaced as the amount you estimated (in % as well) for the "Total" monthly pool-water loss/replacement; that specific # minus the number in box one will tell you how much borates (in ppm) has been approximately depleted from the pool at the end of that month. For example, the last time I remembered putting borates in my pool was 3 months ago and the amount was 0 (I never had borates before, however, you cannot use "0" to express the amount you remembered---it won't work that way). Starting that month, I added two boxes of 20 mule team borax to my 12,000 gallon pool---giving my pool only 11.2 ppm of borates in total. Now, I would type in the first box "11.2" and in the next box I would type 1 ppm less (10.2, 9.2, 8.2, and so forth) until it stated that 20-22% (I'm looking for the monthly estimated pool-water loss that I calculated) of the water in my pool should be replaced. The number that made the "Pool Calculator" say that 20-22% water loss/replacement will decrease the borates to the amount that I typed in box 2 (yes, that's how the box equation works) was approximately "9.2", which means that there will be a 2 ppm decrease (I rounded it to make things easier) in borates at the end of the month. How did I find out that it will decrease to -2 ppm? Well, if you minus "9.2" from "11.2" you get "2.2" or "2" rounded to the nearest whole number. After one month of losing 22% of pool water, my borates depleted by approx. 2 ppm, so I had about 9 ppm left. The month later, I remembered adding another box of 20 mule team borax, which lead to a +5.6 ppm increase--- giving me 14.6 ppm of borates in total---so now I would type "14.6" in the first box and 1 ppm less in the second box (13.6, 12.6, 11.6, and so forth) until I reached 20-22% water loss; Losing 22% of the total water in my pool for that month decreased the amount of borates to 12.5 ppm. The third month, I added 4 boxes of 20 mule team borax which gave me approx. 35 ppm of borates (4 x 5.6ppm + 12.5 ppm), and minus 7 ppm from 20-22% water loss/replacement resulted in a total of 28 ppm for the end of that month. This month, which is October, I added another 4 boxes of 20 mule team borax---giving me +22.4 ppm in borates---which increased the amount of borates in my pool to approx. 50.4 ppm. Basically, any "Total" addition of borates happens in the beginning of the month and any "Total" depletion of borates would happen at the end of the month; however, this varies from pool owner to pool owner, which depends on how frequently they add borates, when they add it, and how much water is being replaced/lost/splashed-out daily/weekly/monthly.
In conclusion, if there isn't any place that sells borate test strips/kits nor a pool store that does it for you, then you can always try to guesstimate the amount you have by making some effort to record, compare, and calculate how much of it you might have or might not have in your pool.