For clarification, I only noted the existence of three suppliers, all of which are frequently mentioned in various online forums. I didn't mean to suggest any endorsement any of them, as I have not dealt with any except as an online information source.
It is the buyer's responsibility to assess both the suitability of the product and the desirability of the retail supplier; Jack's post is valuable input to the choice of retailer. Of course, a specific product (such as a particular model of Curt or Atwood jack) is the same regardless of who sells it to you, so product choice and supplier choice are somewhat independent. If you choose an Atwood jack and Harbor Freight doesn't sell it, then I suppose you're not buying it from them; if more than one supplier has it, then you might buy from the cheapest one that will reliably deliver in a suitable time. If Harbor Freight only has junk, you might not want to buy it, because it's junk.
Just based on anecdotal reports and back-of-magazine ads, Harbor Freight certainly does sell some junk, as I'm sure most retailers do. If they don't have any good stuff, then it would make sense to ignore them entirely. I didn't mention Princess Auto because the request is from someone not in Canada; Princess Auto sells lots of stuff that could be described as junk, but they also sell name-brand products and a couple of levels of house-branded tools and equipment, the better line of which are pretty good, and even the lower line of which are suitable for many purposes. If I needed a trailer jack, I would certainly at least check at Princess Auto. I wouldn't want anyone to write off this retailer just because they have a cheap surplus stuff section.
There are, of course, many more suppliers. I only noted some of the suppliers who would not likely be familiar to someone who had so far dealt only with RV-oriented retailers. For trailer components that are not specific to RVs, this type of supplier may make more sense than an RV parts store; they typically have a wider range than more general stores (you can buy some trailer parts at Home Depot). Camping World will sell you a name-brand trailer tongue jack, probably as expensively as anyone on the planet, and they will do it online or in person (if there is one near you - there are none here). If you like dealing with them, they may be a good choice.
Many RV dealers use the same two or three distributors, and typically charge the same high suggested list price as each other... even higher than Camping World. Their web sites often link to the distributor's online catalog, or you can go to some of those catalogs directly, then shop at whatever RV dealer is handy, has stock, or offers a better price. For instance, GO-RV.com - RV On-line Catalog
is the Coast Distribution catalog: Atwood side-wind A-frame tongue jacks are on page 506.