I use NutriGold, 5000iu, get it from amazon.com for $14.95 /360ct. It's very pure and has no "inflammatory" fillers, which are guilty of aggravating many lyme-related sxs like achiness, fatigue and brain fog etc.
My chiropractor is the one that got me on to looking deep into supplements to make sure they are pure. I found these the cheapest and he approved them. My LLMD also approved them.
For example, SamsClub's Vit D is very cheap, but it contains ingredients in it that cause inflammation, among other unnecessary impurities.
If anybody finds something better/cheaper, I'm all ears.. every penny counts. I also ordered their GreenTea supplement.
You can look at the link and see the label goodies.
Also, It is good to know what your Vit D blood levels are. If they are very low, it is completely safe to take as much as 20,000 iu/daily. Again, my chiro told me about this process called "frontloading." You do this for about 3 months to bring your numbers back up, then take 5-10,000/d is completely safe.
I was very skeptical of these astronomical numbers until I did more research. Drs will Rx 50,000 rx doses if necessary, but you can do the same thing here for much cheaper.