Consider clamping or screwing a heatsink in place and using standard and relatively low cost thermal grease.
If this is for a on off and/or does not have to look pretty, consider whether retaining with a cable tie would be acceptable. This may need holes to be made in PCB, but may not. You MAY even be able to slide a thin tie lengthwise under the DIP package.
You can make your own with a goodish chance of success.
Use epoxy of choice and fill (mix) with about 50% of substance liable to make it suitable conductive thermally.
This*might* be graphite, carbon black, metal powder (copper probably good, brass probably good, Aluminum may be excellent).
You can get powdered Aluminum from paint suppliers (or could). As this was the main fuel of the space shuttle solid boosters it may be harder to buy now than it was just over 10 years ago. Filing the handle of an old saucepan may do almost as well :-).The finer the Al powder the greater the oxide coating % so super fine may not be good.
NOW I Gargoyle it - should have done that 1st.
This man claims excellent results making his own from about 3 parts epoxy and 1 part thermal paste - makes sense.
ebay UK has a few offerings
This is GPB2.25 for a small tube with free economy international postage from Hong Kong BUT it doesn't sound like it makes a very strong bond.
ebay also has lower cost adhesive thermal tape. How good it is is tbd.
GBP1.65 free postage one square inch from Manchester - sounds possibly OK
2 GBP from Gloucester - hopefully their dimensions are wrong.
About GBP5 from GB may be OK but getting up
GBP2.88 from Wimborne 35mm x 35mm