redrockthreads.com has a thread conditioner, Thread Magic Thread Conditioner Square With Cutter
Fabric and Thread Care, it's $12.95. It's also available as 2 cubes for $10.95 and 1 round for $10.95.

Nice recent write-up about thread conditioners by Suzy Quilts - appears the benefits are greater with natural threads (i.e. cotton) vs synthetic (polyester) which is why some opinions may vary. I spent a few hours hand sewing last evening with Aurifil 50wt and Thread Magic - worked wonderful! No tangles, No fraying, No resistance through layers of fabric. I think I might also try the Sew Fine conditioner as I've seen it recommended by several quilters that I follow.

Ultimate Guide to Thread Conditioners

To expand on Susie's answer - put the thread on top of the conditioner, put your thumb on top of the thread, and pull the thread with your other hand. This will coat the thread. ;)

Yes, sorry, I didn't mean to insinuate that thread conditioner doesn't work at all. I love it for cotton thread, but I prefer to use Bottom Line for applique because it's so fine, sinks into the fabric, and I can get that ring of pre-wound bobbins in every color. But when I put thread conditioner on it, it felt like I was dragging Velcro through the fabric.

I have Bottom Line on hand and think I will give it a try, sans conditioner, as that may be a better solution all the way around - I like the idea of a thinner thread that will sink into the fabric - thank you! ;)

Thought I would share my findings after hand sewing appliqué this week. I used aurifil 50wt thread with both Thread Magic and then Sew Fine Thread Gloss.

Sew Fine was a recent purchase I made after seeing it promoted elsewhere - it certainly “coated” my thread but I felt a distinct drag when pulling through the fabric layers. No fraying, no tangles but did not allow my thread to move through the fabric smoothly. Back of the small can says ingredients are beeswax, coconut oil & fragrance. BTW - it smells great!

Thread Magic is a product I’ve used for a few years and by comparison works better - for me - nice & smooth movement, no tangles or issues of any kind. The brand does not seem to share its ingredients.

Lastly, I plan to try polyester thread with no conditioner at all and am anxious to see how that feels and looks compared to what I’ve always done.

I use bee wax. It lasts forever and the one I have has it's own container with slots to slide the thread through.

I mentioned that I'm hand quilting a quilt right now. I bought Gutermann hand quilting thread and it's wonderful. I don't even need thread conditioner.
For years I hand quilted exclusively and always used Coats & Clark. It tangled a lot.