I would consult your vet to see if he knows of a product you can use.
The main ingredient in Rearguard is Cyromazine, which is sold in the US for commercial applications under the brand name Larvadex in topical and feed form for poultry.
Hopefully, your vet can advise a product for use on rabbits.
It's a shame you can't get it in the States. Although it doesn't stop the flies laying eggs, it stops the eggs turning into maggots and feeding on the bun, so it's really worthwhile.
Hopefully, there is a similar product available to you!