Have you tried local feed stores/livestock supply stores? That's WAY cheaper than mail order hay (in the US, it's usually in the $7-15 or so range for a bale that's anywhere from around 50 to 120 lbs).
When it comes to the warning "you get what you pay for," feed store hay is the exception to the rule.
Pet stores overcharge to an absurd degree because they CAN. One bunny eats approximately 5 lbs of hay a month; at $5 a lb, that's about $25 a month - anyone who doesn't know better wouldn't think twice about that cost, as it doesn't sound obviously unreasonable... unless you happen to know that you could supply that same bunny with hay for $1 a month through a feed store.
Horse owners won't put up with BS pet store prices because they're *very* obviously absurd if you're feeding a horse (let alone multiple horses). One horse eats 1/2 to 1 (50 lb) bale PER DAY. At $5 a lb, that's $125 a day or $3750 a month. At $10/bale, that's $155 a month.
Most people can easily afford an extra $24 per bunny per month and will gladly pay pet store prices if they believe they're doing what's best for their pet
(and of course, no pet store will tell you that you could save $3000 over a bunny's 10-year lifespan by getting high quality hay by the bale instead of buying their stale rip-off hay). Darn near no one could afford an extra $3600 per horse per month, and that's why horse owners get non-inflated hay prices of $7-15 a bale
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ANY "horse quality" grass hay is suitable for rabbits of all ages (as opposed to cow quality, which can have icky stuff like mildew in it, and as opposed to legume hays like alfalfa/clover which are too high in protein and calcium for adult rabbits)... though with "cereal" types like oat, wheat, rye, etc. you'd want to remove any seed heads (oats or w/e) before feeding the hay as the seed heads contain excess carbs and calories that can cause serious health problems if a bunny eats a lot of them on a regular basis.
Feed stores are pretty common - you don't realize how many there are until you start looking for one!
If you can't find a feed store for some reason, you can try calling stables in your area (pretty much any place that boards horses, gives horseback riding lessons, sells trail rides or anything else that involves having horses on the premises), explaining that you're looking for horse quality grass hay for a pet bunny and asking if you can buy a few flakes (flakes are the smaller chunks that a bale gets broken down into).
Hay can be stored for a year or more as long as it's kept clean, dry and insect-free (preferably in a cooler location rather than a very warm/hot one), so you should be able to use up most or all of an entire bale even with just one bunny
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While I buy mail order hay myself, I'm quick to recommend against it if feed store hay is an option - the stuff I buy is amazingly fresh and half the price per pound of pet store hay *after* shipping... but it's also around
13 times more expensive than feed store hay. Honestly, the ONLY reason to buy mail order hay is if you don't have the means to get to a feed store/transport a bale home or if your bunny is annoyingly picky and refuses to eat the feed store stuff (the latter being my reason for mail-ordering hay).