Reddit reviews Ivermectin Sheep Drench 8 oz. (Packaging May Vary)

If you're comfortable figuring out doses yourself, sure. Heartguard Plus is just ivermectin (the actual heartworm killer) and pyrantel pamoate (the "plus"), both of which are readily available non-script for livestock.

(Of course, the obvious downside to going totally off-label like this is that if you kill your dog, suing the manufacturer will not get very far.)

Just get a livestock dose and subdivide it. If you go the injectable route, 0.5 and 0.3 ml insulin syringes are available at any pharmacy. (I generally use the 1/2" needles rather than the super-thin 1/3", as they're less prone to bending.)

While, in obscure cases, the nature of the carrier matters, normally it's just active ingredients. In some cases, route of administration makes a difference; typically an oral dose might be larger than injectable due to incomplete absorption.

This is some of the cheapest bulk I can find (8 fl. oz at 0.08% is 0.19g of ivermectin), and judging by the product comments and the common sale combined with 1 ml measuring syringes, it's very popularly used by dog owners.

Note that I have not done the math myself and do not vouch for the dosage table in the Amazon comments. It's probably right, but you can check it as easily as I can.

Ivermectin 1% injectable 50 ml for $28.95 is about as cheap as it gets. This much will last even a busy shelter for a very very long time.

(One I put my vet on to was using Baytril 100 orally. Enrofloxacin is very well absorbed orally, so no dose adjustment is required, and 100 ml times 100 mg/ml for $102.95 is = $10.295/g of enrofloxacin. Compare with 100 tablets times 22.7 mg/tablet for $91.95 is $40.5066/g. Or $28.375/g for the largest (136 mg) pills. And it's easier to subdivide doses, too!)

Edit: Here's some dosage advice from a reputable source: 5 mcg/kg PO monthly. They mix ivomec injectable 1% with 99 parts propylene glycol to make a 0.01% stock solution and administer that. (Propylene glycol is safe; the toxic antifreeze you're thinking of is ethylene glycol.) Here's another source.

Pyrantel pamoate is also very easy to come by if you want. It's even available OTC in human drugstores for pinworms.

Worth pointing out that if heartgard, etc. are too expensive for you, purchase some sheep drench ivermectin and dose appropriately with an oral syringe.

https://www.amazon.com/Ivermectin-Sheep-Drench-Packaging-Vary/dp/B002BADTYG?th=1&psc=1

or

https://www.tractorsupply.com/tsc/product/durvet-ivermectin-sheep-drench-240-ml

It will go bad before you use the entire bottle assuming a non-hoarding number of dogs. Also ivermectin has a pretty large safety window so you can dose a little on the high side with no worries. The heartworm prevention dosage is about 1/100 of what is used to treat mange.

Here's a good article on dosing, etc.

http://dogaware.com/health/ivomec.html